Expert opinion evidence about surveillance and counter-surveillance techniques is admissible where it describes techniques based on the expert's specialised knowledge, but becomes inadmissible where the expert speculates about the purpose of a person's conduct without identifying the expertise on which the conclusion is based. Where tainted expert evidence is directed at a critical part of the Crown case and it is impossible to assess the extent to which the jury relied on it, the proviso in s 6(1) of the Criminal Appeal Act 1912 (NSW) will not be applied.
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