Defence of Mistake of Fact (WA)

This page explains how the defence of honest mistake of fact may be used by individuals charged with a strict liability offence in Western Australia.

Strict liability offences

A strict liability offence is an offence that does not have a mental element (mens rea). When a person is charged with a strict liability offence, it is unnecessary to demonstrate that they intended to commit the offence or foresaw its commission. However, it is necessary to be able to prove that the person knew the facts that made their actions an offence.

In Western Australia, the defence of mistake of fact is solely applicable to strict liability offences.


Section 24 of the Criminal Code Act contains the defence of mistake of fact.

The provision states that an individual is not criminally liable for their actions or omissions if they acted under an honest and reasonable belief, even if that belief was mistaken, as long as their actions would have been legal had their mistaken belief been accurate.

When is the defence of mistake of fact available?  

A person may rely on the defence of mistake of fact in relation to any strict liability offence. Some examples of these offences are:

  • having persistent sexual contact with a child under 16 (section 321A of the Criminal Code Act);
  • displaying offensive material to a child under 16 (section 204A of the Criminal Code Act);
  • breaching a restraining order (section 61, Restraining Orders Act 1997);
  • speeding offences.

Sexual contact with child

If a person is charged with an offence involving having sexual contact with a child under 16 but can demonstrate that they had an honest and reasonable belief that the child was 16 years old or older, they may utilize the defence of mistake of fact.

It is crucial to note that the belief must be both honest and reasonable, and the reasonableness will be determined by the surrounding circumstances, such as the child’s representations, appearance, and behaviour, as well as the accused’s interactions with the child.

Breaching restraining order

Where a person is charged with breaching a restraining order, the defence of mistake of fact can be successful only if the accused was mistaken about a particular fact that led to the charge. It cannot be utilized if the mistake was regarding the restraining order itself.


When a person is charged with a speeding offence, the defence of mistake of fact may succeed if they can demonstrate that their speedometer was defective, and they genuinely believed that they were driving within the speed limit.

Mistake of law not a defence

Individuals who commit an offence because of an error in their understanding of the law do not have a defence. This is clearly stated in section 22 of the Criminal Code Act.

In instances where an error involves both a mistake of fact and a mistake of law, it will be classified as a mistake of law. Moreover, if an error pertains to a thing or location outlined in the legislation, it will also be categorized as a mistake of law.

Burden of proof

If the accused seeks to rely on the defence of mistake of fact, the onus of raising the defence falls on the accused. This means that the defence will need to provide evidence that the accused had a mistaken belief and of what led to that belief.

Once the defence of mistake of fact is raised, the onus of proof will transfer to the prosecution. They will need to prove beyond a reasonable doubt that the accused didn’t act under a mistaken belief.

It is important to note that the defence of mistake of fact does not automatically constitute a complete defence. The accused will still be held accountable to the extent that they would have been had their mistaken belief been accurate.

Thomas v R

The 1937 decision of Thomas v R provided a successful example of the defence of mistake of fact being used to contest a charge of bigamy. The accused argued that he had genuinely and reasonably believed that his first marriage was invalid due to his wife not having received a decree absolute in her previous divorce. This led him to mistakenly believe that she was still married when he married her, making their subsequent marriage invalid. Latham CJ explained that this belief concerned a matter of fact – whether or not a decree absolute had been issued by the Supreme Court of Victoria. If no such decree had been made, the accused’s former wife would still have been married at the time of the accused’s second marriage, thus rendering it invalid. Therefore, if the accused’s belief had been true, he would not have been guilty of the offence of bigamy.

If you require legal advice or representation in any legal matter, please contact Go To Court Lawyers.


Fernanda Dahlstrom

Fernanda Dahlstrom has a Bachelor of Laws from Latrobe University, a Graduate Diploma in Legal Practice from the College of Law, a Bachelor of Arts from the University of Melbourne and a Master of Arts (Writing and Literature) from Deakin University. Fernanda practised law for eight years, working in criminal defence, child protection and domestic violence law in the Northern Territory. She also practised in family law after moving to Brisbane in 2016.
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