Scott Watson's lawyer has argued in the Court of Appeal on Wednesday that crucial identification evidence that led to his convictions was inadmissible.
Watson was found guilty of the 1997 murders of Olivia Hope and Ben Smart, but he's always maintained his innocence.
It was New Year's Eve, 1997. Olivia Hope and Ben Smart were seen in footage just seven hours before they boarded a yacht with a mystery man.
A jury decided the man was Scott Watson, thanks to what was called "crucial" evidence in the trial.
Ferry taxi driver Guy Wallace, who's since died, dropped off Hope and Smart along with the mystery man to a yacht.
Wallace initially rejected that man was Watson in an earlier interview with police - until he was shown a photo in a montage.
At the trial, he was asked how sure he was.
"Pretty definite," he replied.
On Wednesday, in the Court of Appeal where Watson is attempting to appeal his murder convictions, his lawyer Nick Chisnall KC called Wallace's evidence inadmissible.
"The Jury's hearing of the evidence - that Mr Wallace visually identified Mr Watson when shown montage B, we say was an error that caused a miscarriage of justice," Chisnall said.
He told the court Wallace was shown a photo of Watson early on in the investigation.
"Police concertedly and repeatedly use suggestive practices… to secure Mr Wallace's identification of Mr Watson," Chisnall added.
Wallace only identified Watson in a montage later, with his eyes half-closed.
"Montage B was shown well after there had been media saturation of Mr Watson's image," Chisnall told the court.
The final three witnesses on Wednesday discussed the matter of two strands of hair that were linked to Olivia Hope, found on a blanket inside Watson's yacht.
In his submission - Chisnall said during the trial, the Crown fell short of proving the hairs weren't contaminated.
He argued the case is important for not only the families involved, but also the integrity of the country's justice system.
He'll continue his submission on Thursday.
