Scott Watson's appeal of his double-murder conviction is being heard in the Supreme Court.
Watson was sentenced to life imprisonment for murdering Ben Smart and Olivia Hope in the Marlborough Sounds and has now been behind bars for 27 years.
The Blenheim friends, aged 21 and 17, were last seen stepping off a water taxi onto a stranger's yacht in Endeavour Inlet the early hours of 1 January, 1998, after a New Year's Eve party at Furneaux Lodge.
Their bodies have never been found.
Watson was found guilty of the murders in 1999 after an 11-week jury trial involving about 500 witnesses.
He appealed his convictions after the trial but the application was dismissed. He made another two applications that were unsuccessful before a 2017 bid for a royal pardon was granted, with the case heard by the Court of Appeal in 2024.
It focused on the use of photo montages shown to witnesses ahead of the original trial and the reliability of forensic testing used to show two hairs found on Watson's boat that belonged to Hope.
Watson relied on new expert opinion challenging the reliability of the forensic evidence at trial about the two hairs found on a tiger-patterned blanket aboard his boat.
It also considered whether a photo montage used by police had predisposed witnesses to pick out Watson.
At the original trial, the Crown's case relied completely on the positive identification of Watson by water taxi driver Guy Wallace, who dropped off the young pair to a stranger's yacht in the early hours of New Year's Day.
The court's decision, released last September, found there was no miscarriage of justice in relation to the hair evidence or the identification of Watson by Wallace.
Watson then sought leave to appeal that decision.
The Supreme Court granted the appeal in part, approving only the question of whether the Court of Appeal had been correct to conclude no miscarriage of justice arose from the decision of the trial judge to admit visual identification evidence of Guy Wallace.
The appeal focuses on the use of photo montages shown to Wallace.
Watson's lawyers argue the photo identification process was inherently flawed.
