Investigator-Mediated Contamination and the Lessons from State of Western Australia v Piccioni
Across Australia, criminal lawyers routinely confront the same practical problem:
DNA evidence looks compelling on paper — but something about it doesn’t sit right.
Whether the issue is framed as unfair prejudice outweighing probative value, unreliable evidence, or evidence whose admission would be unfair, the underlying forensic question is the same:
Can the DNA evidence be trusted, given how it was collected, handled, and interpreted?
The District Court decision in State of Western Australia v Piccioni provides a watershed ruling of DNA evidence being refused altogether because that question could not be answered safely.
Helen Roebuck acted as the independent forensic DNA expert for the defence.
Different legal tests — the same forensic question
It is important to be precise about legal frameworks.
- Evidence Act jurisdictions (NSW, VIC, ACT, TAS, NT, Cth) apply s 137, requiring exclusion where probative value is outweighed by unfair prejudice.
- Western Australia, South Australia, and Queensland apply different statutory and common law tests when determining whether evidence should be excluded or refused.
However, despite these differences, courts across all jurisdictions are asking the same forensic questions:
- How reliable is the DNA evidence?
- What assumptions underpin its interpretation?
- Are there realistic alternative explanations for the presence of the DNA?
- Is there a risk the evidence will be given disproportionate weight?
Piccioni shows how those questions can be decisive — even outside an Evidence Act framework.
A landmark decision on investigator-mediated contamination
The decision of the District Court of Western Australia in State of Western Australia v Piccioni [2025] WADC 69, delivered on 3 October 2025, is the first published Australian ruling refusing DNA evidence wholly on the consideration of investigator-mediated contamination.
The judgment, published on AustLII and discussed in the Western Australian Brief, recognises that DNA evidence cannot be assessed independently of the conditions under which it was obtained.
This is not a theoretical concern. It is a practical one.
A case built on DNA — and undone by its handling
The prosecution case relied heavily on DNA recovered from a Smith & Wesson revolver seized during a police search of a storage unit. A mixed DNA profile supported the accused as a possible contributor, with a reported likelihood ratio of approximately 100 billion.
Ordinarily, that statistic would be presented as powerful forensic evidence.
However, an independent review of the audiovisual material revealed extensive failures in contamination control during the search and handling of exhibits, including:
- reuse of gloves between exhibits
- uncontrolled movement of items
- significant portions of the search occurring off-camera
- handling practices inconsistent with accepted forensic protocols
These were not minor deviations. They went to the heart of whether the DNA could safely be relied upon at all.
Investigator-Mediated contamiantion in WA v PICCIONI
The exclusion of DNA evidence in WA v Piccioni was grounded in the court’s acceptance that investigator-mediated contamination created a real risk of indirect DNA transfer. This aspect of the decision has broader implications for how DNA handling practices are scrutinised nationally.
Investigator-mediated contamination and indirect transfer
Investigator-mediated contamination refers to the inadvertent transfer of DNA to an exhibit via the actions of police or other personnel, rather than through the alleged offence.
In Piccioni, the observed handling created a clear and substantial risk that DNA could have been transferred from non-inculpatory surfaces within the storage unit onto the firearm through poor contamination-control practices.
This raised a critical forensic problem:
the DNA result could not reliably distinguish between incriminating activity and innocent indirect transfer.
Competing expert opinions — and the Court’s response
While the prosecution expert accepted flaws in contamination-minimisation techniques, they maintained there was no expectation that DNA transfer had occurred.
At voir dire, Judge Astill rejected that position, finding that:
“the risk of investigator-mediated contamination is real and necessarily affects the reliability and weight to be attached to the proposed DNA evidence.”
His Honour further found that the procedures adopted were not consistent with what was required to avoid contamination and stated:
“Ms Roebuck’s opinion more accurately reflects the significance of secondary DNA transfer … I prefer and adopt her formulation.”
Ultimately, the Court ruled that admission of the DNA evidence would be unfair, and that the prejudice arising from the evidence could not be cured by judicial direction.
The DNA evidence was refused.
The prosecution subsequently withdrew the charges.
Why this matters beyond Western Australia
The significance of Piccioni is not confined to WA.
The decision confirms that, regardless of jurisdiction:
- DNA evidence is not automatically admissible
- strong statistics do not guarantee reliability
- collection and handling failures can fatally undermine probative value
- courts are prepared to refuse DNA evidence where fairness is compromised
Whether the argument is framed under s 137, unfairness, reliability, or discretionary exclusion, the forensic foundation is the same.
The real risk: DNA that looks stronger than it is
Modern DNA profiling is extraordinarily sensitive. A handful of cells can generate a DNA profile. This sensitivity increases the risk that DNA evidence:
- reflects indirect or secondary transfer
- allows hidden assumptions about activity
- invites over-interpretation by a jury
- creates unfair prejudice disproportionate to its true value
Piccioni demonstrates that these are not abstract concerns — they are case-determinative ones.
Why early forensic review matters
Cases involving DNA contamination, transfer, and handling issues often turn on details buried in footage, notes, and procedures, not just laboratory reports.
By the time DNA evidence is framed as “compelling”, positions may already be fixed.
Independent forensic review at an early stage can assist criminal lawyers to:
- identify whether DNA evidence is vulnerable to exclusion or refusal
- assess whether alternative transfer pathways remain realistic
- understand how handling practices affect reliability
- formulate legal argument and position in consideration of the true scientific weight, rather than the converse.
A final observation
Piccioni is not a rejection of forensic science. It is a reminder that DNA evidence is only as reliable as the process that produces it.
Where investigative practices fail to match the sensitivity of modern DNA technology, courts will — and should — intervene.
For criminal lawyers dealing with DNA evidence that appears strong but feels unsafe, Piccioni illustrates why independent forensic scrutiny can change the course of a case.
Helen Roebuck is an independent forensic DNA expert specialising in DNA interpretation, transfer, contamination, and evidentiary limits. She assists criminal lawyers across Australian jurisdictions with assessing whether DNA evidence can safely be relied upon — and whether it may be vulnerable to exclusion or refusal.
Concerned about whether DNA evidence can safely be relied upon in your case?
Where collection, handling, or interpretation issues exist, DNA findings may carry limited probative value and create a real risk of unfair prejudice.
Roebuck Forensics can assist in determining whether exclusion or refusal of the DNA evidence should be considered.