An accused person should be acquitted if when they allegedly offended they honestly and reasonably believed in a state of facts that — if true — would have made their acts innocent: Proudman v Dayman (1941) 67 CLR 536.
The requirement that the mistake be both honest and reasonable can make life hard for the accused. What is a reasonable mistake depends on the circumstances of the case. McConville v Watson  VSC 532 is a useful example.
Mark McConville was convicted in the Magistrates’ Court of three counts of making false statements to obtain payments under the Accident Compensation Act 1993. After a work-related injury, he would visit his doctor and obtain certificates to the effect that he was unfit to perform certain kinds of work for his employer, an air conditioning company. He never told his doctor he had started working for his wife’s company, a rival air conditioning business. The presiding magistrate considered Mr McConvile had acted honestly but not reasonably by omitting to tell his doctor about his other job.
Mr McConville appealed to the Supreme Court. One ground of his appeal was about Proudman v Dayman. He claimed he honestly and reasonably believed he didn’t have to disclose his new work role to his treating doctor.
In dismissing that ground of appeal, Coghlan J said [at 53],
53 His Honour found that the appellant acted honestly, that is, he did not set out to deceive Dr Thompson, and may well have confused her role as a treater with her additional role of being a ‘certifier’, but he was the person obtaining the certificate for the purpose of continuing his payments and he was passing them on to his employer and on to the insurer. It was not reasonable for him to have not appreciated the importance of full disclosure to his doctor of what he was doing since it was patent that she was required to certify his work capacity, which she said was ‘nil’. The appellant said in his record of interview that he did not regard that as being his capacity. It follows as an additional consideration that he was put on notice by what Dr Thompson had certified in the form. I do not accept that disclosure to others associated with the worker could alter that position. It follows that I do not accept that the so-called Proudman v Dayman  HCA 28;(1941) 67 CLR 536 ground has not been made out.
54 I also generally reject the notion that a person cannot mislead by material which is provided honestly, although that matter was not separately argued or relied upon before me. It seems that the Magistrate below accepted that the misleading here was by inadvertence. That was consistent with his dismissal of the fraud charges and the small penalty (with conviction) which he imposed on those charges.
Saunderson v Watson  VSC 497 is another recent example of a belief that may have been honestly held, but was found not to be reasonably held.