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DNA evidence is a powerfully applied prosecutorial tool and can seem conclusive. When DNA appears in a case, it can feel as though the lab results will determine the outcome.
DNA Evidence in Criminal Cases explained
DNA evidence is frequently relied upon in criminal prosecutions.
DNA evidence is not a just a series of test results. It is the outcome of a complex process involving collection, testing, interpretation, and reporting.
When someone is charged and the prosecution refers to DNA evidence in court, it can appear that the scientific results will determine the outcome of the case.
Understanding how DNA evidence is evaluated in court is critical to understanding what the results may - and may not - prove.
How DNA Evidence Is Used in Court
Most people first encounter DNA evidence after they have been charged or when reviewing the police brief of evidence.
This follows the investigative stage, when police collect samples from a crime scene, items, or from individuals for comparison.
The samples are then tested and summarised in a police DNA report. That report becomes the first part of the evidentiary DNA material relied upon by the prosecution to support the charges.
By the time a defendant first hears about “DNA evidence”, the early testing has occurred and the results may appear consistent with the allegation.
Following the police DNA report, there will often be further DNA testing as the prosecution proceeds.
The prosecution must serve a “DNA expert report” if the DNA is to be relied upon at trial.
It is important to understand that the DNA expert report may be substantially different to the early police DNA report and it may arrive close to trial.
This can be particularly relevant to anyone considering early plea on the face of a police DNA report, which may be significantly different to DNA evidence upon which the prosecution ultimately relies.
What It Means If You Are Charged With DNA Evidence
When a person is charged and DNA evidence is disclosed, the results can appear decisive.
However, a DNA result does not automatically prove the allegation.
DNA testing shows that genetic material from a person may be present on an item or location.
It does not determine:
how the DNA was deposited
when it was deposited
whether the contact relates to the alleged offence
For this reason, DNA evidence must always be interpreted alongside the circumstances of the case.
A DNA database match occurs when a profile obtained from a crime scene is compared with profiles stored in the national DNA database (NCIDD).
A match does not mean the circumstances of the allegation are proven. It means that the genetic profile obtained from the sample is consistent with a profile already recorded in the system.
The NCIDD Database holds samples from:
Convicted persons
Suspects of current crimes
Forensic samples retained from earlier investigations
Volunteers
If a match is obtained is an investigative lead only. If prosecution proceeds in reliance upon the database match, an evidentiary reference sample must be obtained from the person of interest.
A reference DNA sample is taken from an individual. This might be used for comparison with DNA recovered from a crime scene or item.
This is usually obtained through:
Buccal/mouth swab
Blood sample
The purpose is not to determine guilt, but to allow scientists to assess whether a person can be included or excluded as a contributor to the DNA profile.
The comparison is statistical and does not explain how the DNA came to be present.
Crime scene work is inherently difficult to control for contamination and DNA transfer.
Scenes may be compromised from earlier policing or paramedic activity.
In some jurisdictions, uniformed general duty police may be handling items at scene.
In other jurisdictions, highly trained CSI officers will be conducting such work.
The way a sample is collected, stored, and handled can influence what is ultimately detected. DNA can transfer between people and objects and be present from earlier contact.
Certain matters require a detailed review of crime scene logs, witness statements and BWC camera footage to assess DNA anticontamination procedures applied in the matter.
In sexual assault matters, items such as clothing and bedding are most commonly sampled by police within their facilities.
Following this, the samples are sent to the DNA lab for testing.
The movements of the evidentiary items prior to the sampling counter are also important.
Frequently complainants provide clothing sometime after the alleged incident, which can be problematic for DNA testing. Any DNA upon those items may be transferred from one location to another during handling and packaging.
An evidentiary review of the sampling will consider the sampling officers contemporaneous notes, statement, relevant complainant statement and Crown Case Statement.
It is commonplace for the complainant to be medically examined in a sexual assault.
This process seeks to obtain background information and biological material associated with the allegation.
The process typically uses a specially designed kit for collection of samples by the medical officer, and these are known by various names across Australia, including SAIK, FMEK, FECK, or rape kit.
Broadly, the medical examiner might take swabs of skin or genitals, including internal areas.
These procedures must be taken with the utmost care to avoid DNA transfer from one location to another.
A review of the medical sampling will consider the medical officers contemporaneous notes and statement.
Laboratory testing involves extracting DNA from the samples and generating a DNA profile.
The DNA profile (or profiles) are then evaluated further.
Biological fluid samples may also be subjected to various testing protocols.
The DNA interpretation process is complex and may involve:
Estimating the number of DNA contributors
Consideration of differential extraction lysis
Re-amplifying low level results
Creating composite DNA profiles
Comparing results to numerous ethnicity groups
Interpreting partial profiles
Running many statistical comparisons
In a typical matter, the DNA casefile generated is in the order of 200 pages (excluding the raw data).
The police investigative report is early evidence and might identify DNA and or body fluids.
The report information is used to target the investigation further and to support charges.
Body fluid results are often only presumptively tested and have not been confirmed by testing.
This report does not typically evidence the statistics (ie 100 billion times more likely).
Further DNA testing is often conducted after this report.
This report may describe:
Where samples were collected
Where DNA was identified
Who the DNA might originate from
This report is not a scientific interpretation and is unverified. This report will essentially be replaced by a lab DNA report for trial.
The laboratory report summarises the scientific results.
This report may be served very close to the trial date.
Further supplementary reports may be evidenced.
The report typically includes:
Whether the defendant can be included or excluded
There will frequently be unknown DNA contributors listed
The DNA likelihood ratio (ie 100 billion times more likely)
Biological fluids testing results
The full laboratory casefile will contain information about subjective decisions and assumptions applied.
Commonly, there is other testing in the DNA casefile which is not in the report.
The DNA casefile is not usually provided with the lab report.
The matter cannot be reviewed scientifically without the DNA casefile.
DNA evidence will essentially form part of trial, unless it’s admissibility is challenged.
Studies acknowledge that jurors risk placing undue weight upon DNA evidence.
Undue weight can be increased where there is misunderstanding.
The court must be given DNA evidence that it understands.
The weight applied to DNA by the court depends on:
Whether there are issues with the results
How the interpretation is explained
How the DNA evidence is explained
How the DNA fits into background circumstances
Whether the DNA results support the allegation or not
Where DNA is central, it must be handled with the greatest precision.
DATABASE DNA MATCH
A DNA database match occurs when a profile obtained from a crime scene is compared with profiles stored in the national DNA database (NCIDD).
A match does not mean the circumstances of the allegation are proven. It means that the genetic profile obtained from the sample is consistent with a profile already recorded in the system.
The NCIDD Database holds samples from:
Convicted persons
Suspects of current crimes
Forensic samples retained from earlier investigations
Volunteers
If a match is obtained is an investigative lead only. If prosecution proceeds in reliance upon the database match, an evidentiary reference sample must be obtained from the person of interest.
REFERENCE DNA SAMPLE
A reference DNA sample is taken from an individual. This might be used for comparison with DNA recovered from a crime scene or item.
This is usually obtained through:
Buccal/mouth swab
Blood sample
The purpose is not to determine guilt, but to allow scientists to assess whether a person can be included or excluded as a contributor to the DNA profile.
The comparison is statistical and does not explain how the DNA came to be present.
CRIME SCENE SAMPLES
Crime scene work is inherently difficult to control for contamination and DNA transfer.
Scenes may be compromised from earlier policing or paramedic activity.
In some jurisdictions, uniformed general duty police may be handling items at scene.
In other jurisdictions, highly trained CSI officers will be conducting such work.
The way a sample is collected, stored, and handled can influence what is ultimately detected. DNA can transfer between people and objects and be present from earlier contact.
Certain matters require a detailed review of crime scene logs, witness statements and BWC camera footage to assess DNA anticontamination procedures applied in the matter.
CRIME ITEM SAMPLES
In sexual assault matters, items such as clothing and bedding are most commonly sampled by police within their facilities.
Following this, the samples are sent to the DNA lab for testing.
The movements of the evidentiary items prior to the sampling counter are also important.
Frequently complainants provide clothing sometime after the alleged incident, which can be problematic for DNA testing. Any DNA upon those items may be transferred from one location to another during handling and packaging.
An evidentiary review of the sampling will consider the sampling officers contemporaneous notes, statement, relevant complainant statement and Crown Case Statement.
MEDICAL EXAMINATION
It is commonplace for the complainant to be medically examined in a sexual assault.
This process seeks to obtain background information and biological material associated with the allegation.
The process typically uses a specially designed kit for collection of samples by the medical officer, and these are known by various names across Australia, including SAIK, FMEK, FECK, or rape kit.
Broadly, the medical examiner might take swabs of skin or genitals, including internal areas.
These procedures must be taken with the utmost care to avoid DNA transfer from one location to another.
A review of the medical sampling will consider the medical officers contemporaneous notes and statement.
LAB TESTING
Laboratory testing involves extracting DNA from the samples and generating a DNA profile.
The DNA profile (or profiles) are then evaluated further.
Biological fluid samples may also be subjected to various testing protocols.
The DNA interpretation process is complex and may involve:
Estimating the number of DNA contributors
Consideration of differential extraction lysis
Re-amplifying low level results
Creating composite DNA profiles
Comparing results to numerous ethnicity groups
Interpreting partial profiles
Running many statistical comparisons
In a typical matter, the DNA casefile generated is in the order of 200 pages (excluding the raw data).
POLICE DNA REPORT
The police investigative report is early evidence and might identify DNA and or body fluids.
The report information is used to target the investigation further and to support charges.
Body fluid results are often only presumptively tested and have not been confirmed by testing.
This report does not typically evidence the statistics (ie 100 billion times more likely).
Further DNA testing is often conducted after this report.
This report may describe:
Where samples were collected
Where DNA was identified
Who the DNA might originate from
This report is not a scientific interpretation and is unverified. This report will essentially be replaced by a lab DNA report for trial.
LAB DNA REPORT
The laboratory report summarises the scientific results.
This report may be served very close to the trial date.
Further supplementary reports may be evidenced.
The report typically includes:
Whether the defendant can be included or excluded
There will frequently be unknown DNA contributors listed
The DNA likelihood ratio (ie 100 billion times more likely)
Biological fluids testing results
The full laboratory casefile will contain information about subjective decisions and assumptions applied.
Commonly, there is other testing in the DNA casefile which is not in the report.
The DNA casefile is not usually provided with the lab report.
The matter cannot be reviewed scientifically without the DNA casefile.
EXCLUSION OF DNA EVIDENCE
DNA evidence will essentially form part of trial, unless it’s admissibility is challenged.
Studies acknowledge that jurors risk placing undue weight upon DNA evidence.
Undue weight can be increased where there is misunderstanding.
The court must be given DNA evidence that it understands.
The weight applied to DNA by the court depends on:
Whether there are issues with the results
How the interpretation is explained
How the DNA evidence is explained
How the DNA fits into background circumstances
Whether the DNA results support the allegation or not
Where DNA is central, it must be handled with the greatest precision.
What a DNA Result Actually Tells You
A DNA result is not a conclusion about what happened. It is a scientific comparison between genetic material recovered from one location and a reference profile.
The result may indicate that a person:
may be a contributor, or
is excluded as a contributor.
The DNA profile results reported in the expert report must be expressed using statistical language, for example, “100 billion times more likely”. While this can sound definitive, the statistic describes the strength of the comparison - not how or when DNA was deposited, and not why it is present.
The meaning of the result depends on context, interpretation, and the surrounding circumstances of the case.
How Accurate Is DNA Evidence?
Like any science, DNA evidence is only as accurate as the methods and conditions applied.
In Australia, forensic DNA testing is conducted at a state level.
Each police agency and state laboratory applies its own sampling methods, testing protocols, and reporting practices.
The differing approaches can and do see variation in testing results, reporting outcomes and expert opinions given at trial.
The accuracy and reliability of the DNA evidence in a given matter cannot be established off the face of the DNA expert report alone.
DNA evidence accuracy depends upon:
How items were handled before sampling
Whether samples were contaminated
What items were sampled and where
How many DNA contributors in a DNA profile
What methods are used to test body fluids
Whether body fluids are confirmed
Whether body fluids can be linked to DNA or not
Social contact between parties
How results are reported
For these reasons, DNA evidence involves scientific processes and subjective human decisions.
Accuracy must be considered on the merits of the matter itself.
Questions People Often Ask When DNA Evidence Is Disclosed
Many people searching online simply want DNA evidence explained, particularly when the results appear to support the allegation.
When a DNA result is first disclosed, it commonly raises practical questions, including:
As Andrew Haesler, former District court judge once wrote: “ Like all physical evidence DNA can be used, misused and abused.”
If you are facing DNA evidence, it is critical to understand whether the evidence relies on scientific error, and whether the DNA evidence genuinely supports the allegation.
In certain matters, the DNA results themselves may be correct, but when evaluated correctly, they do not support the allegation. An independent DNA expert review considers these questions.
Why DNA Results Do Not Speak for Themselves
A DNA report explains the scientific comparison, but it does not determine how the DNA came to be present or whether it relates to the alleged offence.
A laboratory report summarises scientific results, but it does not determine:
Whether contact occurred at all
Whether the DNA got there recently
Whether DNA got there by touching
Whether the DNA got there without any contact
How likely the allegations are given the DNA results
Those questions require interpretation.
This is why the same DNA result can be understood differently depending on the circumstances of the case and the human perspective applied.
DNA Evidence in Different Types of Criminal Cases
DNA evidence may arise in many different types of criminal cases including assault, homicide, drugs - firearm possession, and sexual offences.
Some criminal matters involve prior social contact between the individuals involved, which can create particular forensic complexities
Our DNA can be readily deposited on surfaces and people that we come into contact with.
That DNA can then be redistributed to other surfaces or people without us ever contacting them.
DNA may be recovered from:
The body
Clothing
Bedding
Shared environments
The presence of DNA in these contexts does not, by itself, determine when it was deposited or the circumstances in which it came to be present.
Understanding how DNA evidence is evaluated in court is critical to understanding what the results may, and may not, prove
Reviewing DNA evidence early can shape how it is evaluated, interpreted, and ultimately understood by the court.
DNA can be deposited when we touch, sneeze, bleed, sweat, cough or even talk.
The term “touch DNA” can be erroneously applied, because we simply can’t say whether touch occurred.
The term has been replaced with “trace DNA”, meaning DNA which cannot be linked to a particular body fluid.
Trace DNA profiles are common in evidence, because skin cell DNA is common, and it can be difficult to determine whether DNA is from a body fluid.
Trace DNA can be present through:
Direct contact
Indirect contact
Shared environments
The presence of trace DNA does not indicate whether contact occurred or when the DNA got there.
Saliva is frequently relied upon in criminal matters in support of various allegations.
Saliva testing is known to cross react with various substance including vaginal material, urine and sweat.
A positive saliva test result may be informative in some contexts, but it does not automatically explain:
Whether saliva is even present
How the material was deposited
When the material was deposited
Who the material originates from
Whether it relates to the alleged event
Whilst significant issues exist with saliva testing, it should not be presumed that it has no evidentiary value.
Any saliva test positive result should be evaluated carefully in context of the matter.
A positive semen result generally relates to a presumptive test.
It is critical to understand whether semen is truly confirmed.
Confirmation can be conducted through a microscope observation of sperm cells.
A high number of sperm cells might be consistent with a full healthy ejaculate in one matter.
A low number of sperm cells might be consistent with semen transfer in another matter.
Evaluation of semen would consider:
The number of sperm cells
whether semen is consistent with ejaculation
What can be understood about timing
Whether semen is confirmed
Whether the semen can be attributed to an individual
Any semen result is highly scientific and must be reviewed precisely.
Careful scientific review should be applied to any matter involving semen results.
Consideration of blood can rely on visual appearance, which can be problematic.
Reported positive blood often relates to a presumptive test.
This might be because insufficient quantities exist for confirmatory testing.
The significant evidentiary question can be whether the DNA originates from the blood (ie: whose blood is this)
Matching blood to a particular donor can be difficult, particularly as mixed DNA profiles are common.
Review of blood evidence would consider:
Is the evidenced stain appearance consistent with blood
Is testing presumptive and how reliable is that result
Can the stain be linked to a particular donor
Might the stain be consistent with direct or indirect deposit
Certain matters featuring blood may also lend themselves to bloodstain pattern analysis, which might assist with clarifying certain activity related to the staining.
It can be critical to prosecution and defence “whose semen” it is.
To consider “whose semen” the lab will conduct what is called a differential extraction.
The process is not always successful, and it is inherently scientific to interpret.
When reading an expert report with a differential extraction, you will see a “sperm fraction” and a “non-sperm” fraction.
Surprisingly, the “sperm fraction” does not necessarily contain sperm.
Even more surprisingly, the “sperm fraction” and “non-sperm” fraction can be reported when there is no sperm present.
Any differential extraction should be reviewed by a DNA expert.
DNA recovered from clothing and bedding may arise from:
Direct contact
No contact
Secondary transfer
Shared environments
Earlier handling
Later handling
Laundering
The presence of DNA on clothing and bedding does not necessarily indicate when it was deposited or what activity led to its presence.
Interpretation is more than what is merely possible. To consider the DNA profile scientifically, one must evaluate the case background, handling, seizure and sampling applied. BWC footage is becoming an increasingly common inclusion with review of these factors.
TOUCH DNA
DNA can be deposited when we touch, sneeze, bleed, sweat, cough or even talk.
The term “touch DNA” can be erroneously applied, because we simply can’t say whether touch occurred.
The term has been replaced with “trace DNA”, meaning DNA which cannot be linked to a particular body fluid.
Trace DNA profiles are common in evidence, because skin cell DNA is common, and it can be difficult to determine whether DNA is from a body fluid.
Trace DNA can be present through:
Direct contact
Indirect contact
Shared environments
The presence of trace DNA does not indicate whether contact occurred or when the DNA got there.
SALIVA
Saliva is frequently relied upon in criminal matters in support of various allegations.
Saliva testing is known to cross react with various substance including vaginal material, urine and sweat.
A positive saliva test result may be informative in some contexts, but it does not automatically explain:
Whether saliva is even present
How the material was deposited
When the material was deposited
Who the material originates from
Whether it relates to the alleged event
Whilst significant issues exist with saliva testing, it should not be presumed that it has no evidentiary value.
Any saliva test positive result should be evaluated carefully in context of the matter.
SEMEN
A positive semen result generally relates to a presumptive test.
It is critical to understand whether semen is truly confirmed.
Confirmation can be conducted through a microscope observation of sperm cells.
A high number of sperm cells might be consistent with a full healthy ejaculate in one matter.
A low number of sperm cells might be consistent with semen transfer in another matter.
Evaluation of semen would consider:
The number of sperm cells
whether semen is consistent with ejaculation
What can be understood about timing
Whether semen is confirmed
Whether the semen can be attributed to an individual
Any semen result is highly scientific and must be reviewed precisely.
Careful scientific review should be applied to any matter involving semen results.
BLOOD
Consideration of blood can rely on visual appearance, which can be problematic.
Reported positive blood often relates to a presumptive test.
This might be because insufficient quantities exist for confirmatory testing.
The significant evidentiary question can be whether the DNA originates from the blood (ie: whose blood is this)
Matching blood to a particular donor can be difficult, particularly as mixed DNA profiles are common.
Review of blood evidence would consider:
Is the evidenced stain appearance consistent with blood
Is testing presumptive and how reliable is that result
Can the stain be linked to a particular donor
Might the stain be consistent with direct or indirect deposit
Certain matters featuring blood may also lend themselves to bloodstain pattern analysis, which might assist with clarifying certain activity related to the staining.
WHOSE SEMEN?
It can be critical to prosecution and defence “whose semen” it is.
To consider “whose semen” the lab will conduct what is called a differential extraction.
The process is not always successful, and it is inherently scientific to interpret.
When reading an expert report with a differential extraction, you will see a “sperm fraction” and a “non-sperm” fraction.
Surprisingly, the “sperm fraction” does not necessarily contain sperm.
Even more surprisingly, the “sperm fraction” and “non-sperm” fraction can be reported when there is no sperm present.
Any differential extraction should be reviewed by a DNA expert.
DNA ON CLOTHING
DNA recovered from clothing and bedding may arise from:
Direct contact
No contact
Secondary transfer
Shared environments
Earlier handling
Later handling
Laundering
The presence of DNA on clothing and bedding does not necessarily indicate when it was deposited or what activity led to its presence.
Interpretation is more than what is merely possible. To consider the DNA profile scientifically, one must evaluate the case background, handling, seizure and sampling applied. BWC footage is becoming an increasingly common inclusion with review of these factors.
Why DNA Evidence Requires AN INDEPENDENT REVIEW
Forensic laboratory reports are designed to summarise results. They do not explain the subjective decisions, alternative interpretations, or wider consideration applicable to the matter.
The lab DNA report will rarely set out all testing results in the case.
The DNA casefile must be reviewed independently to obtain a full evaluation of the DNA evidence.
An independent review of the DNA evidence involves:
Evaluating how samples were selected and collected (review police sampling)
Reviewing the biological fluid testing
Reviewing medical records relating to collection of intimate samples
Assessing the nature of any DNA mixed profiles (Review electropherograms)
Placing the results within the factual context of the case (review witness statements and background)
Presentation of scientific data relevant to the matter
For these reasons, DNA evidence should be reviewed by an independent forensic DNA expert when its significance is central to the allegation.
What This Means for Your Case
Every criminal matter involving DNA evidence is different.
Some DNA results can be straightforward, but many involve complex scientific decisions, contextual assumptions, and interpretive judgment. Simply reading a lab report does not reveal the full picture of what the evidence may - or may not - support.
Understanding how DNA evidence is generated, tested, and interpreted helps you:
Make informed decisions in discussion with your lawyer
Appreciate the strengths and limits of the scientific material
Ask the right questions about the evidence presented
Recognise when further independent review could be valuable
Having a clear grasp of these elements before key decisions are made - such as admissions, pleas, or trial preparation - can change how evidence is approached and explained in court.
If DNA evidence is central to your matter, careful scientific evaluation alongside your legal advice gives you the best chance of ensuring that the forensic science is fully and fairly understood.
Helen Roebuck is a strongly credentialed and highly regarded forensic DNA expert, trusted within both the legal and scientific communities. She is the most prolific and judgment-published independent DNA expert witness in Australia
Having reviewed DNA evidence in thousands of indictable matters over the last 25 years including homicide, sexual assault and drugs importation.
Accepting simple and complex Legal Aid and privately funded briefs across Australia, New Zealand and select international jurisdictions.
Published judgments:
WA v Piccioni [2025] – WA District Court
R v Murphy [2024] – NSW District Court
LGBTIQ Hate Crimes Inquiry [2023] – NSW
R v Granaghan [2022] – N Ireland Crown
R v Perre [2021] – SA Supreme Court
R v Ke [2019] – NSW District Court
R v TAL [2019] – QLD Supreme Court
R v Haouchar [2025] – NSW District Court
R v Ahmad [2024] – NSW District Court
Evans v R [2023] – NSW Supreme Court
R v Tsakirios [2021] – NSW District
R v Kabbout [2020] – NSW District Court
In many matters, a short scientific review of the DNA casefile is enough to determine whether the DNA work conducted by the lab genuinely supports the allegation or whether the DNA results support an alternate explanation of events.
welcome
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Gabriel ChipkinBarrister
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Jane MurraySolicitor
'Her evidence in Court was instrumental in establishing the deficiencies in the Crown's DNA case, and in obtaining a great outcome for our client’.
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Gilbert Aitken Barrister
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‘’Helen was an excellent DNA expert witness in a trial I ran in the Supreme Court of the Northern Territory’.
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‘I worked closely with Helen during the lead up to one of South Australia's longest running murder trials during 2021’.
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