An accused who, at the time of the alleged offence possessed an honest and reasonable positive belief in a state of facts which, if true, would have rendered their acts innocent, is entitled to an acquittal: Proudman v Dayman (1941) 67 CLR 536.

After the majority decision in CTM v The Queen [2008] HCA 25, it is also clear that there must be an evidential basis for the claim. This does not necessarily have to take the form of calling the accused as a witness, though this may be the most readily available (and persuasive) source.

The mistake must be one of fact. If mistakes of law also exonerated an accused the criminal law would quickly become unworkable. Professor Glanville Williams famously said that the fact that ignorance of the law offers no excuse, “is often almost all of the knowledge that some people have of the law.” In Bergin v Stack (1953) 88 CLR 248, Webb J said that if mistakes of law were accepted as offering a defence, “astounding results could follow.”

Determining what is a mistake of fact and what is a mistake of law is not always easy. The task is made even more difficult when the error is a combination of both factual and legal elements. In Thomas v R (1937) 59 CLR 279, Dixon J (Latham CJ and Rich J agreeing) drew on English authorities to conclude that such mixed mistakes should be considered mistakes of fact. However, more recently Gleeson CJ (Clarke JA and Lee AJ agreeing) decided that mixed mistakes of fact and law will not ordinarily constitute mistakes of fact: Strathfield Municipal Council v Elvy (1992) NSWLR 745 at 751.

A thorough examination of this complex topic can be found in the judgment in Mei Wong Su v Australian Fisheries Management [2008] FCA 1485 at [72] – [87]. In determining charges of illegal fishing, Reeves J embarked on a useful summary of the previous cases.