The Supreme Court has reserved its judgment in whether to grant Mark Lundy leave to appeal his double murder convictions.
Lawyer for Mark Lundy, Jonathan Eaton QC, said Lundy was a "guinea pig of novel and junk science".
"Lundy has been the victim of junk science, and is still a victim of it," Eaton said.
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He said the trial was not fair.
At issue were spots of what were believed to be human brain or central nervous system tissue found on a polo shirt believed to have been worn by Lundy on the night of the murders.
Eaton said the conviction relied on mRNA evidence, which has never been used before, or since.
"It also relied on IHC (immunohistochemistry), which hasn't been used before or since," he said.
Eaton told the court "the assessment of inevitable guilt was flawed".
But Philip Morgan, who appeared for the Crown, said Eaton overstated the effect of mRNA evidence on the trial.
"It was the combination of central nervous system tissue and DNA evidence which was the cornerstone of the Crown's case," he said.
He said the defence case wouldn't have been argued any differently without the mRNA.
Convicted double murderer Mark Lundy was convicted in 2002 of the murders of his then wife Christine and 7-year-old daughter Amber.
In August 2000 their bodies were found bludgeoned, with multiple wounds, at their Palmerston North home.
Christine was found naked in her bed. Amber was found in the hallway, suggesting she'd been fleeing her parents' bedroom.
Police never found the murder weapon, but pathologists claimed a tomahawk had been used, in a frenzied manner, against the victims.
Lundy, a salesman, has always maintained his innocence and that he was away on business in Petone at the time of the murders.
In 2002, Lundy was convicted of the murders and sentenced to life in prison with a minimum non-parole of 17 years.
He appealed his conviction to the Court of Appeal. That appeal was rejected and the non-parole period increased to 20 years.
In 2013, he took his case to the Privy Council in Britain, where his convictions were quashed and a re-trial ordered.
Lundy was again found guilty in a 2015 trial.
He appealed a second time but in 2018 the Court of Appeal released a decision to dismiss the appeal.