Where the substantive requirements of s 108C of the Evidence Act 2008 (Vic) are satisfied — specialised knowledge, opinion based on that knowledge, and the opinion could substantially affect the assessment of credibility — leave should ordinarily be granted, and the occasions for refusing leave to adduce relevant expert credibility evidence in the defence case in a criminal trial would be rare. The s 108C threshold test ('could substantially affect') itself establishes the importance of the evidence, so that a separate importance assessment under s 192 is generally unnecessary unless there are countervailing considerations of such weight as to require exclusion. Expert evidence on the effects of poly-substance abuse on memory reliability goes beyond common knowledge and is admissible under s 108C.
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