Teina Pora's convictions for the rape and murder of Aucklander Susan Burdett 23 years ago have been quashed by the Privy Council in London.
In delivering the judgement tonight, Lord Kerr said the Judicial Committee of the Privy Council has concluded "there was a risk of a miscarriage of justice if Mr Pora's convictions were allowed to stand".
The Privy Council noted in its formal decision the now famous confessions Pora made to police.
It found: "The combination of Pora's frequently contradictory and often implausible confessions and the recent diagnosis of his FASD [fetal alcohol spectrum disorder] leads to only one possible conclusion and that is that reliance on his confessions gives rise to a risk of a miscarriage of justice. On that account, his convictions must be quashed."
The board is seeking submissions within four weeks on the issue of whether there ought to be a third trial for the 39-year-old.
Pora's lawyer Jonathan Krebs says his team will be working on those submissions "and naturally our argument will be that there should be no retrial".
Pora spent 20 years in prison for the rape and murder of Ms Burdett who was bludgeoned to death with a softball bat in 1992.
In 1994, aged 17, he was found guilty of the murder. But in 1999 his first conviction was quashed after DNA evidence linked serial rapist Malcolm Rewa to the attack. Then, in 2000, Pora was again found guilty in a retrial. Pora has always maintained his innocence and was released on parole in April last year.
His lawyers took the case to the Privy Council and in November last year his team presented evidence to a panel of five judges including New Zealander Dame Sian Elias.
Pora's defence had presented new information at the hearing about his mental state, saying his mental age was between nine or 10 at the time of crime, he forgets what he has said and changes his answers to fit what he thinks the police want.
New information also came out in the hearing that Mr Rewa has erectile dysfunction. Pora's defence argued Mr Rewa would therefore have been unlikely to have offended with another person who could have seen the dysfunction and the steps he took to address that.
Police say they will take time to fully consider the judgement and expect to be consulted by Crown Law regarding any Crown submissions on a retrial.
Police noted the comments of the Privy Council in relation to Mr Pora's confession, saying: "In the present case it is clear that none of the police officers exerted pressure on Pora.
"Indeed, they were, if anything, fastidiously correct in their treatment of him."