Convicted double killer John Barlow was granted leave today to have an appeal heard by the Privy Council in London.
Barlow was found guilty at his third trial of the 1994 shootings of businessman Eugene Thomas and his son Gene in their central Wellington office. Jurors failed to reach verdicts in the first two trials.
He was jailed for a minimum of 14 years and is due to be considered for parole for the first time in October this year.
Mr King said today's hearing took over two hours and the Crown argued that the Privy Council didn't have jurisdiction to entertain the appeal because of decisions already made about the case by New Zealand's Governor General.
"It's a very complicated and complex point raised by the Crown involving unprecedented legal submission, he said.
"So what the Privy Council have done is agreed to hear the full appeal, but they will also hear further argument on the jurisdiction issue."
Mr King said there was now a lot of work to be done in relation to the appeal, which was not likely to be heard until next year.
Barlow lost his first appeal in the Court of Appeal in 2006.
In March this year he was denied a second appeal despite forensic evidence used to convict him being called into doubt.
Governor-General Anand Satyanand, on advice from the Justice Ministry and Justice Minister Annette King, refused Barlow's appeal to have his case heard again.
Earlier at Barlow's third trial, the prosecution called a United States Federal Bureau of Investigation (FBI) ballistics expert who provided forensic evidence using lead content tests to match crime scene bullets with those in a box belonging to Barlow.
The FBI has since stopped its bullet-testing practice after scientific criticism it might produce a high rate of false matches of bullets.
Only a fraction of New Zealand criminal cases have ever been granted full Privy Council hearings.
"I think it's huge. Constitutionally it's huge and historically it's very significant as well," he said.
Barlow's wife Angela said today she was thrilled with the news.
"Greg has been battling for four years without pay or legal aid and we are so grateful to him for all his efforts," she told Radio New Zealand. "We've been battling for 14 obviously."
The Barlow case is the third double murder case Mr King has taken to the Privy Council following the Bruce Howse and Scott Watson cases.
Leave was granted for an appeal case to be heard against Howse, who was convicted of murdering his stepdaughters in Masterton in 2001, but a retrial was not granted.
In 2003 Mr King unsuccessfully applied to appeal the Scott Watson case.
Watson was convicted and jailed for the 1998 killings of Ben Smart and Olivia Hope.