This article was written by Fernanda Dahlstrom - Content Editor - Brisbane

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.

Honest And Reasonable Mistake


The defence of honest and reasonable mistake of fact applies only to strict liability offences. Strict liability offences do not require the prosecution to prove that the accused intended to commit the crime in order to find them guilty. They only require the court to be satisfied that the accused committed the act. A person charged with a strict liability offence can raise the defence of honest and reasonable mistake of fact. It is the only defence available to them.

The defence of honest and reasonable mistake will succeed if the accused can demonstrate that they held a positive belief in a state of affairs that, had it existed, would render the defendant’s act innocent.

Onus of proof

If an accused seeks to rely on the defence of honest and reasonable mistake of fact, the defence must raise this as a defence. However, once the defence has been raised, the onus lies on the prosecution to prove that no such belief was held by the defendant.

When may the defence of honest and reasonable mistake succeed?

The defence of honest and reasonable mistake may be successfully argued in the following situations:

  • Where a person charged with having sex with a child under the age of 16 had an honest and reasonable but mistaken belief that the child was 16 or older;
  • Where a  person charged with tax fraud can show that they honestly and reasonably believed that the incorrect information they provided to the tax office was correct;
  • Where a person is charged with speeding but can show that the speedometer in their car was faulty and they honestly and reasonably believed they were travelling below the speed limit.

Thomas v The King

In the 1937 High Court decision of Thomas v The King, the court held that it was a defence to a charge of bigamy that the accused had believed “bona fide and on reasonable grounds” that he was not married and therefore was single and entitled to marry. The basis of the belief of the accused was that his marriage to his “former wife” was not valid because her decree of divorce had not been made absolute so that she was still a married woman when he married her.

In upholding the defence of honest and reasonable mistake, 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.”

Mistake of fact vs mistake of law

It is important to note that a person who commits an offence because of a mistake of law does not have a defence. The defence of honest and reasonable mistake is arguable only in relation to mistakes of fact. If a person commits an act thinking (mistakenly) that it is not a criminal offence, they are guilty of the offence regardless of the mistake. Thus the common saying ‘Ignorance of the law is no defence.’

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

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