David Bain could be free early next week after the Privy Council quashed his 12-year-old conviction for murdering five members of his family.
His lawyers are seeking have him released on bail while authorities here decide if he should face a retrial.
The Privy Council last night said there had been a substantial miscarriage of justice but it was up to the Crown in New Zealand to decide whether Bain should be tried again.
Attorney-General Michael Cullen today said Crown Law would carefully consider what action would be taken in light of the Privy Council judgment.
"The decision is solely for the Solicitor-General to make and there will be no further comment," said Dr Cullen.
His lawyer, Michael Reed QC, said it was unlikely Bain's bail application would be heard today, given the amount of preparation needed.
"We don't know when the bail will be heard. I imagine it will be Monday or Tuesday. I don't think it will be today.
"We have got to get some papers ready, we have got to find a judge," Mr Reed said.
Bain, now 35, has maintained his innocence since being found guilty in May 1995 of murdering his mother, Margaret, father Robin, sisters Arawa, 19, and Laniet, 18, and brother Stephen, 14, in the family's Dunedin home in 1994.
Lawyers for Bain in March laid out nine arguments why his convictions should be quashed and the Privy Council last night agreed.
"In the opinion of the board, the fresh evidence adduced in relation to the nine points summarised above, taken together, compels the conclusion that a substantial miscarriage of justice has actually occurred in this case." It said the convictions should be set aside.
"For all these reasons the board concludes that, as asked by the appellant, the appeal should be allowed, the convictions quashed and a retrial reordered. The appellant must remain in custody meanwhile."
Bain will have to wait in Christchurch Prison until a decision is made on a fresh trial.
"The order of the board for a retrial does not of course restrict the duty of the Crown to decide whether a retrial now would be in the public interest," the Law Lords said in their written decision, handed out after a very brief court hearing last night in London.
They said where issues were not fully and fairly considered by a trial jury, it was not the role of appeal courts to decide whether Bain was guilty.
"At any retrial it will be decided whether the appellant is guilty or not, and nothing in this judgment should influence the verdict in any way."
The Privy Council made detailed comments on each of the nine points raised by Bain's lawyers.
Among reasons for finding there had been a substantial miscarriage of justice was that there was substantial doubt whether Bain would have been convicted if evidence discovered post-trial had been put to the jury.
Robin Bain's mental state was discussed, with mention of journals found in his school office containing stories of the mass murder of a family.
"Many of those facts are highly contentious and the evidence could well have influenced the jury's assessment of them."
Robin Bain's potential murder motive was also discussed. "If the jury found Robin to be already in a state of deep depression and now, a school principal and ex-missionary, facing the public revelation of very serious sex offences against his teenage daughter they might reasonably conclude that this could have driven him to commit these acts."
In New Zealand, Detective Superintendent Malcolm Burgess said police would examine the detail of the Privy Council decision before deciding how to proceed. He said he could not comment on the specifics of the case as the matter was now sub judice.
The Bain case was the final New Zealand one ever to be heard by the Privy Council, which has been replaced by the new Supreme Court.
In 1994, Bain was arrested four days after making a frantic 111 call from the family home. Police responding to the emergency found him huddled in the house babbling incoherently. All five of his family members had died from .22 gunshot wounds to their heads.
Bain was sentenced to life imprisonment with a minimum non-parole period of 16 years.
He has argued that his father killed the family then himself while he (David) was out doing his early morning paper run.
At his last-ditch appeal to the Privy Council in March, Michael Reed, QC, told the five Law Lords that Bain could not have committed the murders because bloody sock-prints found at the murder scene were too small to fit his feet and had to belong to his father.
Robin Bain had been identified as "clinically depressed" and perhaps "psychotic" and had a motive because he was in an incestuous relationship with a daughter and was likely to lose his marriage, career and freedom if found out.
* June 1994: David Bain charged with murdering five members of his family.
* May 1995: High Court jury finds Bain guilty. He is later sentenced to at least 16 years' jail.
* December 1995: Court of Appeal dismisses Bain appeal.
* May 1996: Petition to Privy Council seeking leave to appeal fails.
* June 2000: Aspects of case referred back to the Court of Appeal.
* September 2003: Court of Appeal hears case and decides retrial is not needed.
* June 2006: Bain's legal team wins right to a full Privy Council hearing.
* March 2007: Five-day Privy Council hearing in London begins.
* Last night: Privy Council quashes convictions and says NZ authorities can order a retrial if they wish.