The Supreme Court has turned down Mark Lundy's bid for a third trial for the murders of his wife and 7-year-old daughter nearly 20 years ago.

Photo: RNZ / Sharon Lundy

Lundy was convicted in 2002 of murdering wife Christine and daughter Amber but a retrial was ordered by the Privy Council in 2013.

At the retrial in 2015 he was again found guilty and his life sentence, with a 20-year non-parole period, reinstated.

The convictions were upheld by the Court of Appeal last year.

In a final bid to clear his name at the Supreme Court in August, lawyer Jonathan Eaton QC had argued the entire 2015 trial had been distorted by inadmissible evidence.

The argument hinged on conflicting scientific evidence over stains found on Lundy's shirt, with one test showing it was brain or spinal cord tissue that could have come from an animal, and the other finding the tissue was more likely to be human in origin.

The second test, known as mRNA testing, was found inadmissible by the Court of Appeal in 2018.

However, in a unanimous judgement released this morning, the Supreme Court judges have ruled admission of the mRNA was "not a fundamental error" and did not render the trial unfair.

"The Crown case rested on the claim that Mr Lundy had his wife's brain on his shirt, but that claim did not depend on the mRNA evidence.

"It depended rather on central nervous system tissue and Mrs Lundy's DNA being found together."

The disputed mRNA evidence would have excluded Lundy's argument that the stains were animal fat from cooking, but that defence "had no prospect of success in any event".

Justice Mark O'Regan said the disputed evidence "did not cause Mr Lundy to stay out of the witness box or alter the defence case".

The Supreme Court said it was "also satisfied of Mr Lundy's guilt beyond reasonable doubt" and the tissue on his shirt could only have come from his wife's brain.

His guilt was underscored by other evidence, including traces of blue and orange pain - the same paint used by Lundy to mark his tools - found on the bodies; his daughter's blood was on his shirt and there was evidence the murders had been staged to look like a burglary gone wrong.

'Not the end of the road' - Lundy's lawyer

In a statement issued through his lawyer Jonathan Eaton QC, Lundy continued to maintain his innocence and vowed to fight on.

"Naturally Mr Lundy, his legal team and his supporters who have all worked so tirelessly to both establish his innocence and uphold his absolute right to a fair trial, are very disappointed with the decision of the Supreme Court to dismiss his appeal against conviction," Mr Eaton said.

"It is now well established although perhaps not fully comprehended by the public, that evidence that should never have been put before the jury featured at both his trials. The prosecution of Mark Lundy has been something of a testing ground for new or novel science never previously advanced in any court of law."

Mr Eaton said it was "frustrating and very disappointing" that Lundy had never had the opportunity to have his guilt or innocence determined by a jury "who were not exposed to bad science".

The legal process had stopped Lundy from raising some issues during his appeal, but the newly-established Criminal Cases Review Commission would not be so constrained, he said.

"Whilst the decision of the Supreme Court marks the end of the formal legal appeal processes for Mr Lundy, there is no doubt that his supporters, including forensic experts from around the world, will continue to monitor international developments and will not give up the fight on behalf of Mr Lundy.

"This is not the end of the road in Mr Lundy's fight to establish he has suffered a miscarriage of justice."

Craig Maxwell, Lundy's estranged brother, said he was relieved by the court decision.

It was extremely stressful for him and his family every time the case was in the public domain, he said.

Maxwell had one message to Lundy - let it go.