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Mistake of Fact (WA)

When a person is charged with a strict liability offence, they may rely on the defence of honest mistake of fact. A strict liability offence is an offence that does not involve a mental element or ‘mens rea’ and there is no need to prove that the accused intended to commit the offence. This page outlines the defence of mistake of fact in Western Australia.


In WA, the defence of mistake of fact is contained in section 24 of the Criminal Code Act. Under that provision, a person who does or omits to do something under an honest and reasonable but mistaken belief in a state of affairs is not criminally responsible except to the extent that they would be if that state of affairs had actually existed.

Offences where mistake of fact applies

The defence of mistake of fact applies to strict liability offences only. In Western Australia, this includes the following:

In the above examples, a person charged with an offence involving sexual contact with a child under 16 may rely on the defence of mistake of fact if they honestly and reasonably believed that the child was aged 16 or older. Importantly, the mistaken belief must be both honest and reasonable. Whether a mistaken belief about age is reasonable will depend on all the surrounding circumstances, including representations made by the child about their age, the child’s appearance and behaviour, and how the accused came into contact with the child.

When a person is charged with breaching a restraining order, the defence of mistake of fact may succeed where the accused was mistaken about one of the facts giving rise to the charge, but not where they were mistaken about the restraining order itself.

When a person is charged with a speeding offence, the defence of mistake of fact may succeed if the accused can prove that their speedometer was faulty and that they believed they were traveling below the speed limit.  

Mistake of law not a defence

A person who commits an offence due to being mistaken about the law does not have a defence. This is spelled out in section 22 of the Criminal Code Act.

When a mistake involves both a mistake of law and a mistake of fact, it will be considered a mistake of law. When a mistake involves a thing or place described in legislation, it will be considered a mistake of law.  

Onus of proof

When the accused raises the defence of mistake of fact, the defence has the evidentiary onus. This means that the defence will need to adduce evidence that the accused held a mistaken belief and of what contributed to that belief.

Once the defence of mistaken belief has been raised, the onus of proof shifts to the prosecution. The prosecution must prove that the accused did not act under a mistaken belief.

If the defence of mistake of fact is successfully raised, this will not necessarily amount to a full defence. The accused will still be liable to the extent that they would have been liable had the mistaken belief been correct.

Thomas v R

In the 1937 case of Thomas v R, the defence of mistake of fact succeeded in relation to a charge of bigamy. In that case, the accused argued that he had believed bona fide and on reasonable grounds” that his marriage to his first wife had been invalid because the decree of divorce she had received on a previous marriage had not been absolute. The accused argued that he had mistakenly believed that she was still a married woman when he married her and therefore their marriage was not valid. Latham CJ said: “The belief was that a decree absolute had not been made by the Supreme Court of Victoria. Whether or not such a decree had been made was a question of fact. If no decree absolute had been made, the marriage of the accused’s former wife would not have been dissolved and therefore, she would still have been a married woman when she married the accused. Thus, her marriage to the accused would have been invalid, and he would not have been a married person when he went through the ceremony of marriage with Miss Deed. Thus, if his belief as to the matter of fact mentioned had been true, he would not have been guilty of the offence charged.”

If you require legal advice or representation in any legal matter, please contact Armstrong Legal. 

Fernanda Dahlstrom

This article was written by Fernanda Dahlstrom

Fernanda Dahlstrom has a Bachelor of Laws, a Bachelor of Arts and a Graduate Diploma in Legal Practice. She has also completed a Master’s in Writing and Literature. Fernanda practised law for eight years, working in criminal defence, child protection and domestic violence law in the Northern Territory and in family law in Queensland.

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