A Mapp motion is a formal application asking a court to exclude evidence that was obtained unlawfully or improperly. In Australia, the term is borrowed from the landmark United States Supreme Court case Mapp v Ohio 367 US 643 (1961), but the underlying principle has its own distinct form in Australian law. Here, the core mechanism is section 138 of the Evidence Act 1995 (NSW), which gives courts a discretion to exclude improperly obtained evidence. Knowing how that discretion works can make a real difference to the outcome of your case.
What "improperly obtained" actually means
Evidence is improperly obtained when it is gathered in contravention of an Australian law, in breach of the Australian Constitution, or in a way that would have been unlawful if done by a law enforcement officer. The most common scenarios involve police conducting a search without a valid warrant, breaching the rules governing the detention and questioning of suspects, or obtaining a statement without informing the person of their right to silence.
The breach doesn't have to be deliberate. A technical failure, such as a warrant that doesn't properly describe the premises to be searched, can be enough to trigger a section 138 application. The seriousness of the breach is a separate question from whether it happened at all.
How Australian courts balance the competing interests
Section 138 doesn't automatically exclude improperly obtained evidence. Courts start from the position that the evidence should be admitted, then ask whether admission would bring the administration of justice into disrepute. The burden falls on the party seeking exclusion to persuade the court otherwise.
The factors a court weighs include:
- The probative value of the evidence (how important is it to proving the charge?)
- The seriousness of the breach and whether it was deliberate or reckless
- The availability of other means to obtain the same evidence lawfully
- The impact on the defendant's rights and the gravity of the conduct that caused the breach
There's no formula. A court can exclude critical evidence if the breach was serious enough, or admit minor tainted evidence if excluding it would gut the prosecution case for a trivial procedural error.
When the application is made: the voir dire
A Mapp-style exclusion argument in Australia is almost always run as a voir dire, a mini-hearing held within the trial but in the absence of the jury. The defence calls witnesses, typically police officers involved in the search or interview, and tests the circumstances under which the evidence was gathered. The prosecution responds. The judge then rules on admissibility before the jury hears anything about the contested material.
Getting the timing right matters. A section 138 application made too late, after the evidence has already come before the jury, is unlikely to achieve exclusion and may leave the judge with nothing to do but direct the jury to disregard what they've already heard.
Common situations where the argument arises
The most frequent contexts for an exclusion application in NSW criminal practice are:
- Searches of vehicles, premises, or persons without a valid warrant or proper authorisation
- Police interviews conducted after a defendant invoked their right to silence
- Statements obtained from a defendant who was not cautioned, or who was denied access to legal advice before questioning
- Electronic surveillance conducted without the required court authorisation
Records of interview taken in breach of the LEPRA caution requirements sit at the top of this list. A failure to properly inform a suspect of their rights before an interview is exactly the kind of conduct section 138 is designed to address.
What happens if the application succeeds
If the court excludes the evidence, the prosecution must run its case without it. Sometimes that means the whole charge falls over. A drug possession case where the drugs were found during an unlawful search, for instance, may leave the Crown with nothing once that evidence is removed. In other cases, the excluded item is only one piece of a larger evidentiary picture, and the trial proceeds.
Ann Valos Criminal Law assesses exclusion arguments as early as the committal stage, not just on the eve of trial. Identifying a viable section 138 challenge in the voir dire early can influence how the prosecution approaches the matter and, in some cases, leads to charges being withdrawn before a trial begins. Ann Valos Criminal Law has built this kind of pre-trial analysis into every serious matter the firm handles.
It's worth understanding the relationship between an exclusion application and the defendant's broader rights. The right to silence in Australia is one of the key protections that section 138 enforces in practice: if police ignore it during an interview, the resulting statement may be excluded. That protection is only as strong as the willingness of defence counsel to challenge its breach.
The difference from a no-bill application
A section 138 application and a no-bill application are both pre-trial mechanisms, but they operate differently. A no-bill asks the Director of Public Prosecutions to discontinue proceedings entirely, on the basis that the evidence is insufficient to support a conviction. A section 138 application asks the trial judge to exclude specific evidence. The section 138 route may be the only practical option when the prosecution has a technical case but built it on improperly gathered material.
What Ann Valos Criminal Law can do
Ann Valos Criminal Law is an accredited criminal law specialist firm with over 15 years of experience. Ann Valos Criminal Law examines the circumstances of every search, arrest, and interview for potential section 138 challenges as part of the standard case review. Where a breach is identified, Ann Valos Criminal Law prepares the voir dire application, gathers the relevant police documentation, and conducts the cross-examination of officers needed to put the argument before the court properly. If you think evidence against you may have been gathered unlawfully, getting specific legal advice early is essential.