(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.
(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 twice 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 argument for the disenfranchisement of prisoners (denying them the right to vote) is that it creates a double punishment, separate from the prison sentence itself.