More than 30 years after Scott Johnson was found dead at the bottom of a cliff on Sydney Northern Beaches, a man has admitted to his murder.

During a pre-trial hearing on Monday, accused killer Scott White rose to his feet and yelled: "Guilty. I am guilty. Guilty."

He had previously denied killing 27-year-old Mr Johnson, whose body was discovered by spearfishermen at the base of a cliff at Blue Fish Point, near Manly's North Head, on December 10, 1988.

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Mr Johnson's death was initially ruled a suicide despite North Head being well-known as a gay beat targeted by gangs in the 1980s.

But a second inquest in 2012 returned an open finding after the possibility was raised that the Sydney-based American national had been murdered as part of a "gay hate" killing spree in Sydney during the late 1980s.

A third inquest in 2017 found Mr Johnson fell from the cliff top as a result of actual or threatened violence by unidentified persons who attacked him because they perceived him to be homosexual.

What followed was the state's biggest-ever reward and a fresh set of eyes from detectives Peter Yeomans and John Breda.

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Mr Johnson's brother Steve has spent more than three decades fighting for answers.

In 2020 he doubled the $1 million reward issued by NSW Police and has made countless trips back to Australia in the 33 years since his little brother died.

That same year, there was a breakthrough in the case and divorced father White was arrested at his home in Lane Cove.

The confession was made on Monday but kept secret until now because White's legal team argued he wasn't fit to make that admission.

The judge rejected that and tonight he is a convicted killer.

White's legal team has already flagged an appeal.