Criminal Code Act 1899

A person can not be punished for doing or omitting to do an act unless the act or omission constituted an offence under the law in force when it occurred; nor unless doing or omitting to do the act under the same circumstances would constitute an offence under the law in force at the time when the person is charged with the offence.

If the law in force when the act or omission occurred differs from that in force at the time of the conviction, the offender can not be punished to any greater extent than was authorised by the former law, or to any greater extent than is authorised by the latter law.