Investigator-Mediated Contamination and the Lessons from State of Western Australia v Piccioni
Different legal tests — the same forensic question
- 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.
- 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?
A landmark decision on investigator-mediated contamination
A case built on DNA — and undone by its handling
- reuse of gloves between exhibits
- uncontrolled movement of items
- significant portions of the search occurring off-camera
- handling practices inconsistent with accepted forensic protocols
Investigator-Mediated contamiantion in WA v PICCIONI
Investigator-mediated contamination and indirect transfer
Competing expert opinions — and the Court’s response
Why this matters beyond Western Australia
- 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
The real risk: DNA that looks stronger than it is
- reflects indirect or secondary transfer
- allows hidden assumptions about activity
- invites over-interpretation by a jury
- creates unfair prejudice disproportionate to its true value
Why early forensic review matters
- 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.