Sydney cliff murder conviction set aside

The brother of an American mathematician found at the bottom of cliffs at Manly in 1988 says he's disappointed with a decision quashing the conviction of the accused killer.

Scott Phillip White had long denied murdering Scott Johnson, only to say "guilty, I am guilty" in a surprise plea during a pre-trial hearing in Sydney in January.

He later told his confused lawyers "I didn't do it but I'm saying I'm doing it ... it's the only way, she's going to come after me" - suggesting his former wife would attack him.

White also pointed to stress including seeing the victim's brother Steve Johnson and police in court.

The NSW Court of Criminal Appeal on Friday found the Supreme Court judge had erred for legal reasons in refusing to allow White to withdraw the plea later that morning.

The court said there was a triable issue, "raising a real question of (White's) culpability for the murder" of Mr Johnson.

His conviction and subsequent jail term were set aside.

Steve Johnson said his family was shaken by the decision.

"Certainly, my family and I are all incredibly disappointed - crestfallen - that we haven't found justice for my brother yet after 34 years. We were hoping that we'd all be able to rest after today," he told AAP.

"But we respect this process. I know even my brother Scott would appreciate the care that is being taken.

"The time is frustrating, but we are not going to stop. Whatever lies ahead, we're ready for it."

Mr Johnson said he was lucky to have a good team working on the case and remained ready to give White his day in court.

The matter was remitted to the Supreme Court for mention on December 1 where a plea and trial directions are possible.

Despite the decision, White still needs to apply to overturn his plea.

But the appeals court noted their decision on Friday may cause the NSW Director of Public Prosecutions to reconsider her opposition to that application.

The DPP told AAP it was considering the decision.

White's guilty plea - and whether it was in the interests of justice to permit him to reverse it - was the subject of the two-day hearing in the NSW Court of Criminal Appeal in October.

His lawyers argued the primary judge used a "miscarriage of justice" test to reject his plea withdrawal application when the "interests of justice" test, which is broader, was the correct test.

The Crown had argued if the "interests of justice" test had been applied by the primary judge, the same result was "inevitable".

But the appeal court disagreed, detailing numerous considerations.

White's plea was "entirely unexpected" such that he hadn't received explanations his barrister was mandated to give, the Crown hadn't articulated precisely how it was putting it case and his reasoning for pleading "did not necessarily serve his interests".

He was also cognitively impaired, hadn't spoken to his lawyers about sentencing consequences and almost immediately applied to withdraw the plea.

About 30 minutes after he uttered the words "I am guilty", White signed a statement that told of his confusion, stress and that "I maintain that I didn't cause Scott Johnson's death. I want to confirm my plea of Not Guilty."

Those words were unequivocal, the appeal court said.

"The speed with which the plea was sought to be withdrawn has a number of consequences. It is consistent with confusion on the part of (White)," the court said.

The mathematician's death was initially ruled a suicide, but a coroner in 2017 determined it involved human intervention, leading to White's arrest in 2020.

After a judge refused to allow White to reverse his plea, the 52-year-old was sentenced to at least eight years and three months in jail.

While not finding it was a gay hate crime, Justice Helen Wilson said White had reckless indifference to Dr Johnson's life when he threw a punch near the unguarded edge of a high cliff and fled without notifying police he had disappeared over the edge.