Prisoners sentenced to less than three years in jail have had their voting rights restored.
The prisoner voting bill has passed its third and final reading at Parliament.
But, in a bizarre twist - National supported part of a Green Party amendment which makes the bill inconsistent.
The Green Party put forward two changes to the bill - the first, which would see all prisoners given the right to vote, was voted down.
However, the National Party voted in favour of a change which removes the Electoral Commission's power to stop disqualified voters from enrolling.
Justice Minister Andrew Little said the move was "mindless politics" from National and will be corrected next week.