A listening device warrant issued to investigate an offence that depends on the validity of a statute subsequently declared unconstitutional is itself invalid, because the void ab initio doctrine means the offence never existed in law. A conspiracy to avoid payment of a fee imposed by an unconstitutional statute is not a criminal offence, as it cannot be inimical to the public good to agree to do something which is not a crime. A magistrate's ruling on the validity of a listening device warrant is an interlocutory judgment appealable under s 5F of the Criminal Appeal Act 1912.
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