Expert opinion evidence on automatism caused by prescription drugs such as Stilnox must comply strictly with the requirements for expert evidence: the expert must have appropriate pharmacological expertise (not merely psychiatric expertise), must identify all factual assumptions, those assumptions must be proved by admissible evidence at trial, and the expert must state the scientific criteria enabling evaluation of the opinion. An expert who addresses the accused's capacity to form an intention rather than the voluntariness of conduct addresses the wrong legal question. Police observations of apparently conscious and voluntary behaviour over an extended period carry significant weight in rebutting a claim of automatism and must be adequately addressed by the trial court.
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