The question whether a person was authorised to execute a document is a matter of law dependent on the proper interpretation of the authorising instrument, not a 'state of things' within s 24 of the Criminal Code (WA). The defence of mistake of fact is therefore not available where the alleged mistake concerns the legal effect of an authority document. A co-director's signature authenticating the affixation of a company seal on a mortgage is a material part of the document for forgery purposes because it alters the document's legal effect by making it the company's mortgage.
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