A delusional belief does not, in and of itself, constitute an abnormality of mind within s 23A of the Crimes Act 1900 (NSW). Where a later-diagnosed psychosis is relied upon to support a defence of substantial impairment at the time of the offence, and the inferences of impairment and no impairment are equipoised, the accused's burden of proof on the balance of probabilities cannot be satisfied, and the trial judge is not obliged to leave the defence to the jury. Evidence that an accused sought to avoid liability for the killing is not probative of a capacity to understand right or wrong for the purposes of the mental illness defence.
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