The Government rejected Justice Ministry advice to allow all prisoners to vote, even though it would best uphold human rights and fulfill Treaty of Waitangi obligations.
Instead Justice Minister Andrew Little opted to withhold voting rights from prisoners sentenced to at least three years in prison, which was the law before a 2010 change that applied a blanket voting ban to all prisoners.
The Ministry of Justice said that Little's option would be better than the status quo, but would still be inconsistent with the Bill of Rights Act (BORA) and international human rights treaties that New Zealand has signed.
It would also still lead to a disproportionate affect on Māori, and therefore breach Treaty of Waitangi obligations.
• Prisoners serving sentences of less than three years to vote at 2020 election
• National won't support granting prisoners voting rights
• Premium - Political Roundup: Should we care about prisoners voting?
• Supreme Court says ban on prisoner voting was lawful
Last year the Supreme Court upheld earlier court rulings that the ban on prisoner voting breached the BORA, while a scathing Waitangi Tribunal report found that it had had a disproportionate effect on Māori.
Little responded by saying the law would change to what was before the 2010 change, and that those changes would be in place before this year's September election.
But the Justice Ministry, in its Regulatory Impact Assessment (RIA) released yesterday, said that there was no "good policy rationale" for banning any prisoners from voting.
The RIA noted the 1986 Royal Commission on the Electoral System, which had some sympathy for the widely held view that some prisoners should forfeit the right to vote.
"Possible rationales that have been suggested include punishment, deterrence, or breach of social contract," the RIA said.
"We do not consider any of these provide a good policy rationale for disqualifying prisoners from voting. Imprisonment is the punishment, and there is no merit in also removing electoral rights.
"There is no evidence that suggests disqualifying sentenced prisoners from voting deters people from committing crimes."
The RIA added that overturning the ban completely would best encourage electoral participation and prisoner rehabilitation, and be the fairest option given the Waitangi Tribunal's report into the impact of the 2010 law change on Māori.
The tribunal's report found that Māori were 11.4 times more likely to be disqualified from voting than non-Māori, but a threshold of a three-year sentence would still see Māori 2.1 times more likely to be disqualified.
Ministry of Justice estimates show that 2010 change has seen an additional 27,000 people removed from the electoral role - most of them Māori.
The RIA said that a return to the pre-2010 law would also breach the International Covenant on Civil and Political Rights, and may also breach the International Convention on the Elimination of All Forms of Racial Discrimination.
A bill to restore voting rights to prisoners serving a sentence of less than three years' jail has been introduced to Parliament.
The Government wants to pass it in time for those affected to be able to vote in the September election.
The Herald is seeking comment from Little, who is overseas at the moment.
Prime Minister Jacinda Ardern has previously said that a threshold of a three-year jail sentence meant a prisoner would be able to vote on the Government that would be in power when they were released.
"We accept that prison does strip away a person's freedoms for a period of time. I don't think anyone would deny that is one of the roles as part of paying that price to society," she said last year.
"The balance is about right at three years."
The National Party has said that if the Government changed the blanket ban, it would reinstate it.