Teina Pora's legal team say they have a huge amount of work before their appeal is heard by the Privy Council in London.
Lawyers want him released on bail now the court has agreed to hear an appeal against his conviction for the 1992 rape and murder of Susan Burdett in her South Auckland home.
Pora, 38, has twice been convicted of raping and murdering Ms Burdett and has spent 20 years in jail for the crimes he says he did not commit.
There has been growing momentum from justice advocates, community campaigners, politicians and even police officers to have the case re-examined.
His lawyers last August filed an appeal with the London-based Privy Council, which on Friday announced it had granted the application.
Prime Minister John Key said everyone had the right to use the courts to try to prove their innocence and that was a strength, not a weakness, of the justice system. Some significant issues about the case had recently come to light, he said.
"Everyone has the right to continue to test whether they are innocent or guilty. If they believe that they are innocent and can put up a genuine case, which has certainly been the situation here, where the Privy Council has given leave for Teina Pora's counsel to take his case back, and let's see what the Privy Council says."
One of Pora's lawyers, Jonathan Krebs, said the appeal would be heard principally on the reliability of Pora's confession to police, and also on a specific piece of evidence relating to Malcolm Rewa, who has been convicted of Ms Burdett's rape.
Mr Krebs told Radio New Zealand's Morning Report programme on Friday he and private investigator Tim McKinnel, who has worked in the case for five years, visited Pora in prison on Friday morning to tell him the news.
"He was momentarily stunned, I think would be the fair way to describe it, but absolutely delighted. It's been a long time coming. He's been waking up every morning hoping that there would be news."
Mr Krebs said he expected to apply for bail on Pora's behalf and would work through that process. The team had no legal aid and it was not clear whether they would get it. They would have to do a huge amount of work to prosecute the appeal and could have to fund it themselves, he said. Another option was relying on donations.
Mr McKinnel, a former police officer, told Morning Report it was pretty well accepted within the police that Pora's case was troublesome.
"It's an unusual case in that so many people who had some inside knowledge about it were uncomfortable with it and didn't have to scratch very deep to in my view become pretty concerned about it."
Brother wants police review
The brother of Susan Burdett is suggesting that there be a review of the investigation by police, as happened in the Arthur Allan Thomas case, and it should be carried out before the Privy Council hearing.
Jim Burdett believes Teina Pora is innocent and said he hoped the absurdity of his convictions would be obvious to the Privy Council.
Mr Burdett said the original investigation was flawed. "I believe that there was aspects of the original investigation which were dubious in their value. For example, showing him the house where the murder took place. He couldn't find it, but he had to be shown it. And I don't believe that would happen today, so it needs another look at."
Meanwhile, a former lawyer said he was concerned the appeal will focus on placing the blame on his former client. Barry Hart represented serial rapist Malcolm Rewa, who is in jail for sexual assaults on 25 women, including Susan Burdett. The Privy Council would hear evidence regarding him and Mr Hart said he was worried the appeal would be based on blaming Rewa.
Justice Minister Judith Collins said the decision to allow Pora to appeal was not an indictment on the justice system.
"What it shows is that 20 years after some of these events have taken place there have been advances in forensic science, for instance, which often makes it easier to be able to take cases."
Ms Collins said if the Privy Council found in Pora's favour the case would generally go back to trial and, if it did not, the option remained available for Pora to approach the Ministry of Justice, or her as minister, and the Governor-General to seek a pardon.
She said an incomplete application for a pardon, currently on hold at the request of Pora's legal team, had been lodged.
Pora was convicted in 1994 but the Court of Appeal ordered another trial after DNA from Rewa was linked to the attack. Rewa was found guilty of raping Ms Burdett, but two juries could not decide if he also murdered her.
In 2000, Pora was retried and found guilty of the rape and murder and his appeal to the Court of Appeal was dismissed.
The Privy Council appeal is expected to be heard in the second half of this year, probably October.