We see many clients who often come to us with a criminal charge claiming they did not know their actions constituted a breach of criminal law. In most circumstances, the phrase “ignorance of the law is no excuse” is uttered to dishearten clients when no other defence is available. However, did you know that to some extent, ignorance of the law is a full defence if you can satisfy that you held an honest and reasonable mistaken belief of fact?

Often it is difficult to differentiate between reasonable mistake of fact and law as they are generally intertwined and mirror each other. To give an example which will be used throughout this article, a person who is caught exceeding the speed limit but was unaware that the speedometer in the car was faulty, is that person guilty of the offence? What are the avenues of recourse to defend the matter?


Honest and reasonable mistake of fact is a defence which applies to strict liability offences, which do not require proof of fault, negligence or criminal intent. These types of offences do not require the prosecution to prove that an accused person intended to commit the crime. In other words, the mere commission of an act would satisfy the offence. Some common examples of these type of offences include drive whilst suspended.

CASE LAW: CTM v The Queen [2008] HCA 25

A well-known case which dealt with the defence of reasonable mistake of fact explained its function to strict liability offences– CTM v The Queen [2008] HCA 25. Here, the appellant was convicted by a jury for an offence of sexual intercourse with a person of or above the age of 14 and under the age of 16 years. The appellant reported to the police that he was convinced that the complainant was in fact 16 years old. The appeal raised a significant question of law. The question was whether or not the defence of honest and reasonable mistake of fact applied in the circumstances.

In that case, the appellant failed to discharge the burden of evidence that he was unaware as to the fact in issue, being the victim was 16 years old. The main issue was an insufficient supply of evidence which would support the honesty and reasonableness of the asserted belief. The appellant did not give sworn evidence about his belief and the complainant was not asked to respond to the appellant’s assertion in his interview with police that she had told him she was 16. Both of these evidential issues raised significant obstacles for the appellant to prove the defence applied.


The defence of honest and reasonable mistake of fact is derived from common law which as part of a significant principle, states that a person cannot be criminally liable for an act or omission to act if he or she had an honest and reasonable belief in a state of facts, which if it were true, would have made the act or omission to act innocent.

Therefore, the defence is based on the idea that if an honest and reasonable mistake of fact as to a particular state of affairs actually existed, then no criminal offence was committed. Consequently, the accused will no longer be held liable for the offence.

In raising this defence in evidence to argue strict liability offences, you must be able to prove that you held an honest and reasonable belief that facts existed which, had they actually existed, would have meant you were not committing an offence. This is a wholly subjective test. Once satisfied, the test then becomes objective in proving whether the belief was reasonable in the circumstances.

Applying it to the offence referred to above, if you are charged with exceeding the speed limit, but were not aware that your speedometer was recording an incorrect travelling speed, this gives rise to the defence of honest and reasonable mistake of fact. After satisfying that a mistake of fact existed, the Court will also need to be satisfied that your belief of that mistake of fact was ‘reasonable’. The Court will come to a conclusion on this issue by applying it to the ‘reasonable person test’ which requires an assessment of whether an ordinary person in the same circumstances would have held the same belief.


A person accused of a crime will not be able to argue the defence of honest and reasonable mistake of fact if you did not know what you were doing was illegal. The focus of the defence is whether or not the facts which would have satisfied the offence, arises out of a mistaken belief of fact.

Thus, if the speedometer in your car was not calibrated correctly and you were charged with exceeding the speed limit, you could argue an honest and reasonable mistake of fact as you relied on the faulty speedometer, honestly believing that it was accurate, and if the speedometer had been correct, you would not have committed the offence.

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