Prisoners fighting to regain the right to vote are now planning to take their case to the United Nations.
In a ruling released yesterday, a majority of the Supreme Court found the 2010 law change was passed legally, despite an earlier declaration that it breached the Bill of Rights.
Barrister Richard Francois, who represented jail-house lawyer Arthur Taylor and three other claimants, said last month's declaration was a moral victory but did not actually change anything.
A win yesterday, however, would have instantly overturned the law and restored prisoners' right to vote.
"It's unfortunate it didn't go our way, but we're now going to take it to the United Nations Human Rights Committee and seek relief there. I'm planning to do that before Christmas."
He will argue the law change breached the International Covenant on Civil and Political Rights.
The claim will be based on the right to vote, the right to be free from discrimination on the basis of race (given that Parliament was aware that most prisoners were Māori when it passed the law) and the right to be treated in a humane and dignified manner when incarcerated.
Once it received the committee's report, the government would have six months to "rectify the matter", Mr Francois said.
"And if they don't, then they will face enforcement provisions and measures, which will include expulsion from the General Assembly... which would be highly embarrassing."
The Howard League, a group advocating for prisoners' rights, said it was "disappointed" with the Supreme Court decision but noted it was not unanimous.
In a dissenting judgement, Chief Justice Dame Sian Elias, said she would have allowed the appeal because it concerned a fundamental part of voting law, and protected the right to vote.
The group's spokesperson Christine McCarthy said the ruling "does not change the fact that denying prisoners the right to vote is inconsistent with New Zealand's Bill of Rights".
JustSpeak director Tania Sawicki Mead said just because the law change was legal, did not make it right, as the earlier declaration showed.
"Certainly we would encourage Parliament to take the earlier ruling of the inconsistency with the Bill of Rights seriously and put it on their agenda - particularly given that in opposition many of the current MPs said the law was inappropriate and unnecessary," said Tania Sawicki Mead.
JustSpeak and the Howard League both have parliamentary petitions asking the government to give prisoners' back the right to vote and aim to present them by the end of March.