Criminal Code Act 1983

A person cannot be found guilty of an offence unless the conduct impugned would have constituted an offence under the law in force when it occurred; nor unless that conduct also constitutes an offence under the law in force when he is proceeded against for that conduct.

If the law in force when the conduct impugned occurred differs from that in force at the time of the finding of guilt, the offender cannot be punished to any greater extent than was authorized by the former law or to any greater extent than is authorized by the latter law.