The District Court’s decision in State of Western Australia v Piccioni marks the first time an Australian court has published an exclusion of DNA evidence on the basis of investigator-mediated contamination. The ruling signals an urgent need to reform crime-scene handling practices nationwide.
A landmark decision
The District Court of Western Australia’s recent published decision in State of Western Australia v Piccioni [2025] WADC 69 is the first of its kind in Australia, marking an unprecedented judicial acknowledgement of the risks posed by investigator-mediated DNA transfer. The judgment, delivered on 3 October 2025, is one of the clearest judicial recognitions to date of the real-world risk posed by investigator-mediated DNA transfer, sometimes referred to simply as forensic contamination. It also exposes a recurrent problem: the justice system continues to rely on DNA results without fully accounting for the conditions in which samples are collected, handled and interpreted.
As a forensic DNA expert, I have long warned that DNA’s perceived infallibility can mask systemic vulnerabilities. The Piccioni ruling has finally placed those vulnerabilities front and centre.
A case that turned on DNA — and collapsed because of it
The prosecution’s case in this firearms possession matter relied heavily on DNA recovered from a Smith & Wesson revolver seized during a police search of a storage unit in 2019. The mixed DNA profile supported the accused as a possible contributor with an extremely strong statistical weighting of 100 billion, ordinarily the kind of result presented as compelling forensic evidence.
However, my independent review of the audiovisual footage revealed extensive forensic handling failures – reuse of gloves between exhibits, movement of items without contamination control, and large portions of the search conducted off-camera. It was my opinion that the way the search and the subsequent rendering safe of the firearms were conducted represented a serious departure from accepted contamination-control practice. The handling observed created a clear and substantial risk of investigator-mediated contamination – that is, the inadvertent transfer of DNA from non-inculpatory surfaces within the accused’s storage unit, to the firearm through poor evidence-handling procedures by investigators.
Conversely, having viewed the same footage, the prosecution expert had opined in a written report: ‘Whilst the above outlines flaws in the contamination minimisation techniques employed in this instance, there is no expectation that DNA transfer (contamination) has occurred as a result of such flaws’.
At Voir Dire Judge Astill accepted that, ‘the risk of investigator-mediated contamination is real and necessarily affects the reliability and weight to be attached to the proposed DNA evidence‘ finding that ‘the procedures adopted were not consistent with what was required to avoid investigator-mediated contamination.‘ His Honour specifically stated that ‘Ms Roebuck’s opinion more accurately reflects the significance of secondary transfer … I prefer and adopt her formulation‘.
Ultimately, Judge Astill ruled that, ‘The prejudicial effect of this evidence outweighs the limited probative value it holds. To admit this evidence would result in an unfairness that cannot be cured by direction’
Following exclusion of the DNA evidence, the prosecution discontinued the case. The DNA appeared compelling on paper, but its forensic integrity could not be trusted.
High-sensitivity DNA, low-sensitivity procedures
Modern DNA technology is extraordinarily sensitive: only a handful of cells can generate a full profile. This is a scientific triumph, but also a contamination hazard. The Piccioni ruling highlights what many practitioners already know – frontline investigative procedures have not kept pace with this sensitivity.
Police may enter scenes without anticipating DNA risks. They understandably prioritise safety or speed, but without structured supervision from trained forensic specialists, even well-intentioned actions can facilitate unintended transfer. Gloves, often treated as a universal safeguard, are not. They become reservoirs of DNA within seconds of touching an object. Without systematic glove changes, clean work surfaces and disciplined exhibit-handling protocols, the risk of transferring DNA from item to item is significant. Across jurisdictions, these gaps remain routine.
National implications
This is not merely a Western Australian issue. The decision exposes a national challenge:
- DNA evidence is only as reliable as the process that produces it.
- Investigators – not just scientists – now determine DNA reliability.
- Courts are increasingly prepared to exclude compromised DNA evidence.
The belief that ‘DNA can’t be wrong’ has never been scientifically accurate. Piccioni lays bare the consequences of that misconception.
A needed reckoning
The Piccioni ruling is not a setback for forensic science — it is a long-overdue reckoning. We cannot continue to treat DNA as infallible while tolerating substandard crime-scene practices. If police procedures do not evolve to match the sensitivity of modern DNA technology, more cases will collapse, more evidence will be contested and public confidence in forensic science will erode.
