(1) No one shall be liable to conviction of any offence on account of any act or omission which did not constitute an offence by such person under the law of New Zealand at the time it occurred.
Section 26: NZ Bill of Rights
(2) No one who has been finally acquitted or convicted of, or pardoned for, an offence shall be tried or punished for it again.
This right stops unfair punishment. The State can’t punish you for something that wasn’t a crime when you did it, and you cannot be taken to court again for the same incident once you have been acquitted or convicted.
These protections stop abuse of power through shifting laws or repeated prosecutions.
In a related matter, one reason not to deny prisoners the ability to vote is that the loss of their votes is a double punishment, separate from the prison sentence itself.