Mark Lundy's last-ditch appeal against his convictions for murdering his wife and daughter 19 years ago has wrapped up its first day in the Supreme Court.
Lundy was convicted in 2002 for the murder of Christine and Amber Lundy in their Palmerston North home in August 2000 and sentenced to life in prison.
However, he has always maintained his innocence and an appeal to the Privy Council saw his conviction quashed in 2013 and a retrial ordered.
At the retrial, in 2015, he was again found guilty and his life sentence was reinstated.
After the Court of Appeal upheld his convictions, the Supreme Court agreed to hear an appeal on very narrow grounds, known as the proviso.
The proviso is the mechanism the Court of Appeal used to dismiss Lundy's appeal against his 2015 convictions, even though the court found some of the scientific evidence used was inadmissible.
The Crown case at the retrial asked the jury to consider three scientific pieces of evidence found on the shirt Lundy was wearing at the time of the murders, DNA, central nervous system tissue (CNS) and messenger RNA (mRNA).
The Crown argued these three pieces of evidence proved the tissue on the shirt was his wife's brain.
After the retrial the Court of Appeal ruled the mRNA inadmissible, but applied the proviso which meant it thought the other evidence was still strong enough to justify the guilty verdict.
Lundy's lawyer, Jonathan Eaton QC, today told the court the mRNA evidence was a cornerstone of the Crown's argument.
"And that's a key issue on this trial because the Crown approach, clearly marked by the history, is that the CNS plus the DNA did not get them over the line."
He said the Crown "fought exhaustively" to get the mRNA evidence approved for use at the trial.
Mr Eaton argued that without the mRNA evidence it was unclear what the jury would have decided at the retrial.
He told the court a lengthy pre-trial hearing on whether the mRNA evidence was to be allowed made the retrial unfair because it soaked up a lot of the defence's time.
He also said greater care should have been taken in applying the proviso, because the Crown's case had changed significantly between the first and second trial.
"When you change your theory so significantly ... 14 years after the event, then that causes significant prejudice, significant unfairness to a defendant."
However, the Crown's lawyer Philip Morgan QC told the court the case's history was irrelevant and that the Crown's case hadn't hinged on the admissibility of the mRNA evidence.
"The bulk of the Crown closing address, certainly in the first part, is all about the CNS and the DNA and there's a small passage on mRNA... that was the significance it had in the end - barely touched on it."
He said it would have been an extraordinary coincidence that Lundy could have ended up with CNS tissue and Christine Lundy's DNA - which was consistent with blood or tissue - on his shirt, if he had not killed her.
Mr Eaton will have a chance to respond to Mr Morgan's submissions when the appeal resumes tomorrow.