An honest and reasonable belief in a state of affairs which, if they existed, would render the defendant's act innocent affords an excuse for doing what would otherwise be an offence.
What has to be proved
There is an evidentiary onus (the accused must be able to point to some evidence that raises a mistake of fact) upon an accused to raise the defence of honest and reasonable mistake, but once raised, the onus lies on the prosecution to prove that no such belief was held by the defendant.
Thomas (1937) 59 CLR 279
The High 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 a single man 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.”