Western Australia, Australia

Excluding unlawfully obtained evidence in WA drug cases: the Bunning v Cross discretion

When police find drugs during a search that should never have happened, a common assumption is that the evidence must be thrown out. Australian law does not work that way.

Published 27 October 2026

There is no automatic exclusionary rule. Instead, a court has a discretion to exclude evidence that was obtained unlawfully or improperly, and it exercises that discretion by weighing competing public interests.

The leading authority is a Western Australian case decided by the High Court: Bunning v Cross (1978) 141 CLR 54. Because WA has not adopted the uniform evidence legislation used in several other jurisdictions, the common law discretion from that case remains central to how WA courts deal with unlawfully obtained evidence in drug prosecutions.

The case itself

Bunning v Cross arose from a road traffic matter in Western Australia. A driver was stopped by police and charged with driving under the influence of alcohol. The way police obtained the breath analysis evidence did not comply with the statutory procedure. The magistrate excluded the evidence, and the case made its way to the High Court.

The High Court confirmed that a judge has a discretion to exclude evidence obtained unlawfully or improperly, building on the earlier decision in R v Ireland (1970) 126 CLR 321. It also made clear that the discretion is not exercised to punish police. It involves balancing two public interests:

• the public interest in bringing to conviction those who commit criminal offences; and
• the public interest in protecting individuals from unlawful and unfair treatment by those who enforce the law.

In Bunning v Cross itself, the High Court ultimately found that the circumstances did not justify exclusion of the breath test evidence. That outcome is a useful reminder that establishing an illegality is only the first step.

The factors courts consider

The High Court declined to lay down a closed list of criteria, emphasising that each case turns on its own facts. However, the judgment and later cases identify factors that commonly guide the exercise of the discretion:

Was the breach deliberate, reckless or accidental? Deliberate or reckless disregard of the law by police weighs heavily toward exclusion. An honest mistake, made in good faith, generally weighs less.

Does the illegality affect the reliability of the evidence? If the unlawful conduct casts doubt on the evidence itself, that supports exclusion. Where the evidence is reliable despite how it was obtained, as is often the case with drugs physically found during a search, courts tend to give that significant weight.

How easily could the law have been complied with? If police could readily have obtained a warrant or followed the correct procedure, a decision not to do so can count against admitting the evidence.

How serious is the offence? The seriousness of the alleged crime is relevant to the public interest in conviction. This factor often weighs against exclusion in serious drug matters, though it does not decide the question on its own.

What did Parliament intend? Where legislation deliberately limits police powers, a court can consider whether admitting evidence obtained in breach of those limits would undermine the legislative purpose.

Who carries the burden?

At common law, the accused person bears the onus of establishing the misconduct and persuading the court that the evidence should be excluded. This differs from the uniform evidence legislation adopted elsewhere, where section 138 requires the party seeking to admit illegally or improperly obtained evidence to justify its admission once the impropriety is shown.

In WA courts applying the common law, the defence therefore needs to prove the illegality or impropriety and then build a persuasive case on the balancing exercise. The Australian Law Reform Commission has observed that evidence is not often excluded under the Bunning v Cross discretion, which reflects the difficulty of that task.

Common situations in WA drug cases

The discretion can arise wherever drug evidence is obtained through questionable police conduct. Typical examples include:

• Searches without reasonable grounds. Many WA search powers, including the stop-and-search power in section 23 of the Misuse of Drugs Act 1981 (WA), depend on police having reasonable grounds to suspect something. If those grounds did not exist, the search may have been unlawful. See our guide to police drug search powers in WA.
• Defective or exceeded search warrants. A warrant may be invalid, or police may have searched beyond what it authorised.
• Unlawful access to phones and devices. Digital evidence is increasingly important in supply cases, and the authority for accessing it can be contested.
• Improper questioning. Admissions obtained in breach of the rules governing interviews raise their own issues.

Confessions and the fairness discretion

The Bunning v Cross discretion is one of several grounds on which evidence may be excluded. Admissions made to police are subject to separate rules. A confession must be voluntary to be admissible, and a court also has a discretion to exclude an admission where it would be unfair to the accused to use it. These principles were discussed by the High Court in cases such as R v Swaffield (1998) 192 CLR 159. In practice, a challenge to a police interview may rely on more than one of these grounds.

Police-procured offences: a related but distinct issue

A different set of questions arises where police did not merely gather evidence of an offence, but were involved in bringing the offence about, such as through an undercover purchase or an arranged delivery of drugs. The High Court addressed this in Ridgeway v The Queen (1995) 184 CLR 19, recognising a public policy discretion to exclude evidence of an offence unlawfully procured by law enforcement officers.

In WA, that area is now heavily shaped by the Criminal Investigation (Covert Powers) Act 2012, which governs authorised controlled operations and modifies how courts approach evidence produced by authorised conduct. If your charge arose from an undercover operation, read our detailed guide on how WA law treats entrapment and police-facilitated drug offences.

How a challenge is usually run

An application to exclude evidence is usually heard before the trial proper, often on a voir dire (a hearing within the trial, in the absence of any jury). The process typically involves:

1. Identifying the precise power police relied on and the legal conditions attached to it.
2. Obtaining the documents, including warrants, authorisations, notebooks, body-worn camera footage and running sheets.
3. Testing the officers’ evidence about what they knew, suspected and did, and why.
4. Making submissions on the balancing exercise, addressing each relevant factor.

If key evidence is excluded, the prosecution may be unable to prove its case. If it is admitted, the issues raised may still be relevant to other parts of the defence, or to sentence.

What this means for you

If you have been charged with a drug offence and believe police acted unlawfully, it is worth having that conduct examined, but expectations should be realistic. An unlawful search does not automatically end a prosecution, and courts frequently admit reliable physical evidence where the breach was not deliberate or reckless. The strength of an exclusion argument depends on the details of the police conduct and the evidence that records it.

Advice from a lawyer practising criminal law in Western Australia is the best way to assess whether a challenge is worthwhile in your circumstances. Quantity-based supply charges under the deemed supply presumption are covered separately in our article on possession with intent to supply in WA.

For information about legal services in Perth, visit our Perth team at Podmore Legal — Perth criminal defence specialists in drug offence matters.

Related reading: entrapment and undercover drug operations, possession with intent to supply, excluding unlawfully obtained evidence, police drug search powers, the Virginia criminal law overview, and the TroofLaw blog.