Defendants and their lawyers often discuss whether to plead guilty based on the DNA evidence.

If a plea decision is being considered, it is important to understand the true strength of the DNA evidence.

POLICE DNA REPORTS – BEFORE YOU CONSIDER A GUILTY PLEA

Police DNA reports are often produced early, sometimes at the same time charges are laid.

These results are preliminary only and must be approached with caution.

In NSW preliminary police DNA reports are recognisable by the declaration which states: “this is not an expert statement or certificate.” In other states, these early DNA results might be marked with preliminary report or interim report.

It is critical to understand that the report may not include all work done to date, and there will be further work done if the matter proceeds to trial.

The results are essentially what is necessary to support charges and may be useful investigatively.

The results in such reports are generally not sufficient to evaluate the strength of the DNA evidence.

These preliminary results cannot be relied upon in court.

Any DNA evidence relied upon at trial must be presented as an expert statement or certificate prepared by a forensic scientist.

At the preliminary results stage, critical analysis and evaluation have not been completed.

Note: A DNA “match” in a police report naming you, does not establish that the DNA is yours. Likewise, the report stating “semen identified” does not mean that you deposited semen, nor does it even mean that sperm cells have been detected.

Of course, you may acknowledge the alleged events and therefore the value of the DNA results is essentially irrelevant.

However, if you dispute the allegations, the DNA case file can be scientifically reviewed at this early stage to evaluate what the testing actually does and does not support.

The DNA casefile is a large set of documents produced by the DNA lab, which is not attached to the Police report, but details all the work conducted by the lab in the matter.

A DNA expert review of the casefile material can address questions such as:

- Is there evidence of sperm cells?

- Could any sperm be attributable to you?

- Could it be attributable to someone else?

- What is the statistical strength of the DNA result?

- Are there unknown DNA contributors?

- Does the evidence support that you touched the surface?

- Does the DNA support the allegation, or could it be explained another way?

Preliminary DNA results in police reports - What they do and don't mean

A positive test result for suspected saliva, is known as “presumptive” and it cannot confirm saliva.

– The test result might be a reaction to common substances including vaginal secretions, sweat, faeces, urine or saliva.
– A named person or gender cannot be established by this result.
– The report is not evidence of how the substance got there.

A positive test result for suspected semen, is commonly “presumptive” meaning it is not a confirmation of semen.

– The test result might be a reaction to common substances including some fruits, vaginal secretions, urine, sweat, faecal matter, breast milk or seminal fluid.
– A presumptive positive is not evidence that sperm cells are present.
– A named person or gender is not established by this result.
– The report is not evidence of how the substance got there.

Trace DNA is DNA which is not established to be from a particular cell type.

– Skin cells are a common source of Trace DNA.
– This DNA may be attributable to the named person, at an unknown likelihood.
– This DNA is not proof of it to being from a biological fluid (ie saliva, blood or semen).
– The likelihood of the DNA profile can be as high as 100 billion times more likely. It can be as low as 2 times more likely. The likelihood number somewhere between 2 and 100 billion must be calculated and evidenced for trial and significantly effects the strength of the evidence.
– The report is not evidence of how the DNA got there.

A mixed DNA profile is sample with DNA originating from more than one person.

– This DNA may be attributable to the named person and unnamed persons.
– This DNA is not proof of it to being from a biological fluid (ie saliva, blood or semen).
– The likelihood of the DNA profile can be as high as 100 billion times more likely. It can be as low as 2 times more likely. The likelihood number somewhere between 2 and 100 billion must be calculated and evidenced for trial and significantly effects the strength of the evidence
– The report is not evidence of how the DNA got there.

This is DNA testing which targets male DNA only.

– This DNA may be attributable to the named person and unnamed males.
– This DNA is not proof of it to being from a biological fluid (ie saliva, blood or semen).
– A father, brother and sons share exactly the same YSTR DNA profile.
– The likelihood of the DNA profile can be as high as 5600 times more likely. It can be as low as 2 times more likely. This likelihood number must be calculated and evidenced for trial, and significantly effects the strength of the evidence.
– The report is not evidence of how the DNA got there.

SALIVA POSITIVE

A positive test result for suspected saliva, is known as “presumptive” and it cannot confirm saliva.

– The test result might be a reaction to common substances including vaginal secretions, sweat, faeces, urine or saliva.
– A named person or gender cannot be established by this result.
– The report is not evidence of how the substance got there.

SEMEN POSITIVE

A positive test result for suspected semen, is commonly “presumptive” meaning it is not a confirmation of semen.

– The test result might be a reaction to common substances including some fruits, vaginal secretions, urine, sweat, faecal matter, breast milk or seminal fluid.
– A presumptive positive is not evidence that sperm cells are present.
– A named person or gender is not established by this result.
– The report is not evidence of how the substance got there.

TRACE DNA

Trace DNA is DNA which is not established to be from a particular cell type.

– Skin cells are a common source of Trace DNA.
– This DNA may be attributable to the named person, at an unknown likelihood.
– This DNA is not proof of it to being from a biological fluid (ie saliva, blood or semen).
– The likelihood of the DNA profile can be as high as 100 billion times more likely. It can be as low as 2 times more likely. The likelihood number somewhere between 2 and 100 billion must be calculated and evidenced for trial and significantly effects the strength of the evidence.
– The report is not evidence of how the DNA got there.

MIXED DNA PROFILE

A mixed DNA profile is sample with DNA originating from more than one person.

– This DNA may be attributable to the named person and unnamed persons.
– This DNA is not proof of it to being from a biological fluid (ie saliva, blood or semen).
– The likelihood of the DNA profile can be as high as 100 billion times more likely. It can be as low as 2 times more likely. The likelihood number somewhere between 2 and 100 billion must be calculated and evidenced for trial and significantly effects the strength of the evidence
– The report is not evidence of how the DNA got there.

YSTR DNA PROFILE

This is DNA testing which targets male DNA only.

– This DNA may be attributable to the named person and unnamed males.
– This DNA is not proof of it to being from a biological fluid (ie saliva, blood or semen).
– A father, brother and sons share exactly the same YSTR DNA profile.
– The likelihood of the DNA profile can be as high as 5600 times more likely. It can be as low as 2 times more likely. This likelihood number must be calculated and evidenced for trial, and significantly effects the strength of the evidence.
– The report is not evidence of how the DNA got there.

Early Review of the DNA Case File

A police DNA report does not contain the full forensic picture.
The laboratory case file records the testing, interpretation and scientific decisions behind the result and any result which are not included.

A preliminary independent review of the DNA case file can identify whether the evidence genuinely supports the allegation, or whether important scientific limitations or alternative explanations exist.

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

Helen Roebuck DNA expert giving evidence

'I engaged Helen Roebuck to independently examine a complicated DNA issue arising from a Rape matter. Ultimately, the jury returned a verdict of ‘Not Guilty’ on all counts. We couldn’t have done it without Helen.'

Jasmine Pisasale Director, Slade & Parsons

'a complicated DNA contamination matter. Although the DNA evidence initially seemed damning to the accused, the client was acquitted of all charges. Helen’s advice and evidence at Trial was invaluable.'

Kestin Mildenhall Crockett Chambers

'Helen was engaged on short notice. District Court of NSW trial we were faced with a “word on word” situation for a number of alleged sexual assaults, with very little independent evidence to assist the defence. The result was that the DNA evidence was not led, enabling the matter to be much shorter than would have been the case, which proceeded to “not guilty” verdicts on all counts.

John Gooley Stacks Collins Thompson

‘Her professionalism, dedication and expert knowledge were instrumental in the presentation of the defence case’.

Gabriel Chipkin Barrister

‘Very professional, organised, and most importantly a very impressive witness. I would not hesitate to refer her for any DNA related matters’.

Jane Murray Solicitor

'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’.

Simon Kenny Barrister

‘Her work in unpicking and explaining the intricacies of STRmix including who was and was not a contributor to a complex DNA profile was amazing’.

Gilbert Aitken Barrister

‘Helen’s comprehensive report resulted in the prosecution determining not to adduce the DNA evidence’.

Andrew Culshaw Barrister

‘Helen's report was the " final piece of the puzzle " and without it we would not have been able to prove our case’.

Michael Lee Lawyer

‘The first person I called was Helen to thank her for preparing me for a successful cross-examination’.

Maeve Curry Barrister

‘’Helen was an excellent DNA expert witness in a trial I ran in the Supreme Court of the Northern Territory’.

Gabriel Chipkin Barrister

‘Opinion was detailed, concise and persuasive. I highly recommend Helen Roebuck’.

Amelia Ramsey Lawyer

‘I worked closely with Helen during the lead up to one of South Australia's longest running murder trials during 2021’.

Gilbert Aitken Barrister

'I’d absolutely recommend seeking her guidance and opinion'

James Leaver Samuel Griffith Chambers

Helen Roebuck CV