A defence of honest and reasonable mistake under s 24 of the Criminal Code (WA), when founded upon a system rather than personal knowledge of a particular transaction, requires a system designed to eliminate rather than merely minimise errors. A system that the defendant knows produces errors, even if it reduces their frequency, cannot found a reasonable belief that any particular transaction complied with the statutory requirements. The defence of mistake is not impliedly excluded by statutory duties expressed in absolute terms such as a duty to 'ensure' compliance.
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