National and Labour are blaming each other for a law change that means a prisoner will have to be advised of their right to vote - even if they can't vote.
Justice Minister Andrew Little now plans to introduce a new bill to fix the unworkable law next week.
The passing of the Electoral (Registration of Sentenced Prisoners) Amendment Bill last night means that prisoners serving sentences of less than three years will be able to vote, including in September's election.
It passed its remaining stages under urgency with the support of Labour, NZ First and the Greens, but MPs heatedly debated the bill after it was amended at the committee stage.
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The Greens tried to change the bill so that all prisoners would be granted the right to vote. A clause to enable that was voted down, but a secondary clause was passed when National supported the Greens and voted for it.
That clause requires the prison manager to collect prisoner enrolment information and pass it to the Electoral Commission, even though the commission can't enrol prisoners serving a sentence of three years or more.
"So we've now got an absurd situation where ... corrections officers are required to tell all prisoners that they are entitled to vote, but only some of those will actually be entitled to do so," National MP Chris Penk said last night during the bill's third reading.
Act leader David Seymour referred to the Government tabling the wrong bill last month.
"This is the biggest legislative screw-up by this Government under urgency for, oh, about four weeks since we accidentally passed the wrong law and unwittingly legislated a $2.5 billion business loan scheme because the Government couldn't put the right bill on the Table.
"So now we have a situation where prisoners are going to be enrolled regardless of how long they are sentenced. But it's not clear whether being enrolled will actually allow them to vote or by whether enrolling them to vote means the prison warden will have actually committed a crime."
In a statement this morning, Little blamed what happened on "mindless politics from the National Party".
He will introduce another bill to clear up the law next week. It may have to pass under urgency to be in place for September's election.
National's electoral law spokesman Nick Smith said National had acted with integrity and had simply voted for a clause it supported in principle.
He said it was the Government's fault that the law was now "shambolic and unworkable".
"If the normal parliamentary process had been applied, any gaps would have been caught and fixed.
"National attempted to have the bill referred back to the Justice Committee to correct the errors. The Government refused that offer but now admits the new law will have to be amended before the ink is dry.
"The Government is too focused on trying to score a few thousand more votes from prisoners at the 2020 election rather than providing robust, well-thought through electoral law."