DNA EVIDENCE: ADMISSIBILITY AND UNFAIR PREJUDICE
DNA evidence is often treated as objective, definitive, and difficult to challenge.
In practice, the admissibility and weight of forensic DNA evidence is nuanced, matter-specific, and continually evolving. The real issue is rarely “does the DNA match?” – it is whether the evidence is being advanced in a way that is scientifically justified, logically valid, and fair in the context of the trial.
Admissibility challenges involving DNA evidence are rarely won on broad assertions. They turn on detail: the underlying raw data, the interpretive settings and assumptions, and whether the conclusion extends beyond what the science can support.
Roebuck Forensics assists criminal lawyers by providing independent forensic DNA review to support matters where the defence is considering whether to exclude DNA evidence, limit its use, or challenge the way it is being presented – including in the context of voir dire preparation and probative value versus unfair prejudice considerations (including s 137 Evidence Act in Uniform Evidence Act jurisdictions).
Common issueS in matters where exclusion is considered:
In cases where exclusion is in issue, the following scientific concerns commonly arise:
• Low-level DNA findings where uncertainty and variability increase
• Mixed profiles requiring interpretive decisions and assumptions
• STRmix or other probabilistic genotyping results being overstated beyond source level
• Propositions that do not reflect the real dispute in the case
• DNA transfer, persistence or contamination risks that are not properly addressed
• Weak scene context or uncertainty about the biological source of the DNA
Where these issues exist, the true probative value of the DNA evidence may be materially different from how it appears on the face of the report.
In cases where exclusion is in issue, the following scientific concerns commonly arise:
• Low-level DNA findings where uncertainty and variability increase
• Mixed profiles requiring interpretive decisions and assumptions
• STRmix or other probabilistic genotyping results being overstated beyond source level
• Propositions that do not reflect the real dispute in the case
• DNA transfer, persistence or contamination risks that are not properly addressed
• Weak scene context or uncertainty about the biological source of the DNA
Where these issues exist, the true probative value of the DNA evidence may be materially different from how it appears on the face of the report.
Probative value vs unfair prejudice (s 137 Evidence Act)
In Uniform Evidence Act jurisdictions, lawyers will be familiar with s 137 of the Evidence Act, which requires exclusion of prosecution evidence where its probative value is outweighed by the danger of unfair prejudice. Comparable exclusion principles apply in non-Uniform Evidence Act jurisdictions (including WA, Queensland and South Australia), although the statutory framework and terminology differ.
Forensic DNA evidence can raise these issues where:
- the statistic carries an outsized “scientific authority” effect
- a source-level conclusion is used to imply activity (“contact”, “use”, “involvement”)
- key limitations and assumptions are not made transparent
- alternative explanations (transfer, persistence, contamination) remain plausible
- the DNA is the primary evidence relied upon by the Crown
My role is not to provide legal advice, but to provide independent scientific analysis that assists counsel to accurately characterise what the DNA evidence does (and does not) establish — so it can be properly framed in conference, cross-examination, and submissions.
When DNA evidence becomes unfairly persuasive
DNA evidence is powerful — but it is not infallible.
Modern forensic DNA interpretation can involve mixed profiles, low-level DNA, probabilistic genotyping (including STRmix), and reporting frameworks that require assumptions and professional judgement. The evidential weight of a DNA finding is therefore rarely self-evident from the face of the report.
The most effective challenges to DNA evidence are rarely about arguing “DNA is unreliable”. They are usually about precision: what was tested (and what was not), what assumptions were required to generate the statistic, what question the statistic actually answers, and whether conclusions are being extended beyond the data.
This level of analysis is time-consuming and highly technical. Independent review can clarify the true probative value of the evidence — and the risk of overstatement or misuse in court.
In practice, careful preparation for an admissibility challenge (including a voir dire) often exposes limitations and vulnerabilities in the way the DNA evidence is being advanced. In some matters, that process results in the prosecution withdrawing or narrowing reliance on the DNA evidence, particularly where probative value is limited or unfair prejudice is a realistic risk.
Case notes (selected published judgments)
Roebuck Forensics has been engaged in a significant number of matters across jurisdictions where DNA evidence has been excluded, limited, or substantially reframed following independent expert review and clarification of the scientific issues. In many cases, those outcomes occur through pre-trial processes and are not publicly reported.
The following case notes are selected published judgments from matters in which Roebuck Forensics reviewed the DNA evidence, and where the Court referred to that expert opinion in its reasoning.
State of Western Australia v Piccioni [2025] WADC 69
A significant voir dire decision involving DNA evidence, including consideration of investigator-mediated contamination risk and the limits of inference that can be drawn from a DNA finding. The judgment provides a clear example of the Court assessing forensic evidence in context, rather than treating a DNA result as determinative.
Judicial comment:
“Ms Roebuck’s opinion more accurately reflects the significance of secondary transfer … I prefer and adopt her formulation.”
— Judge Astill, State of Western Australia v Piccioni [2025] WADC 69
R v Ke [2019] NSWDC 349
A District Court decision arising from allegations of sexual offending against a child complainant. DNA evidence from vulval/labial swabs was excluded under s 137 Evidence Act (NSW) because the result could not resolve the critical issue of how the DNA was deposited (direct deposition versus indirect transfer), and any probative value was outweighed by the danger of unfair prejudice and jury misuse.
Judicial comment:
“I refuse to admit the evidence sought to be adduced by the prosecution because if it does have probative value, that value is outweighed by the danger of unfair prejudice to the accused by reason of the possibility of misuse of the evidence by the jury”
— Judge Grant, R v KE [2019] NSWDC 349 [2025] WADC 69
Why Helen Roebuck
I have worked extensively across a very broad range of matters in which the question of exclusion has been applied to the DNA evidence.
My depth of experience is marshalled to carefully evaluate the particular issues present.
Undoubtedly, the merits of one matter may support entirely different prospects of exclusion than another.
I am uniquely placed to evaluate, decipher and persuasively explain the science in consideration of exclusion where warranted.
Next Step
If you are considering an admissibility challenge, or need independent forensic input to assess whether DNA evidence is being overstated:
A preliminary DNA expert review can be a useful step in assessing the broad strengths and weaknesses of a particular matter. This process can also identify documentary and any further evidence requirements.
Rigorous interrogation of the evidence will uncover underlying issues and determine the most appropriate pathway towards reviewing and reporting the matter.
QCertain complex matters benefit from a draft DNA expert report, which can open areas for discussion with Counsel, and potentially allow for defence to approach the prosecution.
Following a thorough evaluation of the evidence, a DNA expert report will be issued in accordance with the expert witness code of conduct. The report will be suitable for submission in evidence.
Preparations with Counsel are often conducted, such that the probative value of the evidence is weighed effectively and persuasively at Voir Dire, should such a hearing be required.
Extensive preparations are generally conducted in anticipation of substantive hearing. Which may include cross examination and evidence in chief scenarios specific to the matter .
Professional Memberships
International Association of Bloodstain Pattern Analysts
The International Association of Bloodstain Pattern Analysts (IABPA) is a worldwide professional forensic organization for experts in bloodstain pattern analysis (BPA), promoting the science, standardizing techniques, and providing education to help investigators reconstruct crime scenes by interpreting bloodstain shapes, sizes, and distribution to understand the events, force, and weapons involved.
The IABPA unites scientists, law enforcement, academics, and criminal justice professionals to advance this field, which uses physics and biology to interpret patterns for court cases.
Australia and New Zealand Forensic Science Society
The AFS (Australian Forensic Society) was formed in 1971 to bring together scientists, police, criminalists, pathologists, and legal professionals actively involved in the field of forensic science. In 1988, the AFS recognised New Zealand members and changed its name to ANZFSS.
Australian Academy of Forensic Sciences
Chartered Society of Forensic Sciences
The Forensic Science Society was founded in 1959. Now accepting memberships globally the CSOFS is the peak professional body for forensic practitioners, academics, researchers and associated professions in the United Kingdom.
In 2014, the Society was granted a Royal Charter and became The Chartered Society of Forensic Sciences.
The CSOFS is recognised as the ‘international voice of forensic science’.
The Chartered body is committed to integrity and impartiality, aiming to provide opportunities for practitioners and academics to congregate, communicate and collaborate.
Common questions about excluding DNA evidence
Can DNA evidence be excluded in a criminal trial?
Yes. Depending on the jurisdiction and circumstances, DNA evidence may be excluded where its probative value is limited, where it is unfairly prejudicial, or where there is a real risk the evidence could be misused or overstated in court.
What is s 137 of the Evidence Act and how does it relate to DNA evidence?
In Uniform Evidence Act jurisdictions, s 137 requires exclusion of prosecution evidence where its probative value is outweighed by the danger of unfair prejudice. DNA evidence may engage this where the statistic is likely to be misunderstood or relied upon beyond what it can scientifically support.
When is DNA evidence most vulnerable to an admissibility challenge?
Common examples include low-level DNA, complex mixtures, STRmix results with strong assumptions, uncertain biological source, or where transfer/persistence/contamination are realistic alternative explanations.
When might a voir dire be used in relation to DNA evidence?
A voir dire is a pre-trial process where the court determines admissibility issues. It may be used where there is dispute about whether a DNA result should be admitted, limited, or framed in a particular way for the jury.
What is the difference between source-level and activity-level DNA evidence?
Source-level addresses whether a person could be a contributor to the DNA profile. Activity-level addresses how the DNA was deposited (direct contact, secondary transfer, handling pathways, persistence and timing). Confusing these levels is a common cause of overstatement.
Can DNA evidence be misleading even if the lab result is technically correct?
Yes. The result may be scientifically valid, but still misleading in context if key limitations are not disclosed, propositions are too narrow, or the evidence is used to imply activity-level conclusions it cannot support.
How can STRmix affect admissibility arguments?
STRmix is a probabilistic genotyping system that generates likelihood ratios using modelling assumptions and settings. In some matters, admissibility concerns arise where assumptions materially influence the LR, transparency is limited, or the LR is used beyond its proper level of inference.
Do I need an expert to challenge DNA evidence?
Often, yes. Admissibility challenges typically turn on the underlying raw data, the interpretive assumptions, and whether the conclusion extends beyond what the science can support. Expert review can assist by identifying vulnerabilities and explaining them in clear, court-usable terms.
What does Roebuck Forensics review in a DNA exclusion brief?
Depending on the scope, this may include the forensic report, analytical and interpretive documentation, mixture interpretation/STRmix outputs (if applicable), transfer/persistence considerations, proposition wording, and the suitability of the inference being advanced.
Can early expert review lead to DNA evidence being withdrawn?
In some matters, careful preparation for admissibility arguments can result in the prosecution narrowing or withdrawing reliance on DNA evidence, particularly where limitations become clear or the probative value is reduced when properly framed.