After three inquests and almost 30 years, a coroner has found 27-year-old American mathematician Scott Johnson's death in 1988 was the result of a gay hate attack.

The decision comes at a historic time for the LGBTI community in Australia, Mr Johnson's family said, but they cannot ignore that "the killers probably still live among us".

Mr Johnson's naked body was found at the bottom of a 60-metre cliff at North Head, Manly, in Sydney.

"I am of the view it is very unlikely Scott took his own life," coroner Michael Barnes said.

Scott Johnson was said to have the world at his feet and had just been told he would be awarded a PhD. ( NSW Police )

Mr Barnes stressed there were many reports of gangs of men, including soldiers from the nearby Army barracks, converging on the area where Mr Johnson died and committing gay hate assaults and robberies.

The coroner also concluded the area, Bluefish Point, was a well-known gay beat, something police denied during the investigation.

"I am persuaded to the requisite standard that Scott died as a result of a gay hate attack," he said.

The coroner found it was most likely that more than one person was responsible for the attack which led to Mr Johnson either being pushed off the cliff, or him falling while trying to escape.

"There is however insufficient reliable evidence to identify the perpetrators," he said.

He made note that Mr Johnson's wallet was not found with his clothes or at his partner's house where he had been staying.

It was also concluded that although Mr Johnson's clothes were found folded and undamaged near the cliff's edge, he may well have been already naked when he was set upon.

Despite the Johnson's family call for an investigation by the NSW Crime Commission, the coroner declined to make such a recommendation.

"There are over 500 suspicious deaths that are awaiting investigation, none of which have received the same scrutiny this case has," Mr Barnes said.

'The killers still live among us'

Mr Johnson's brother Steve thanked the inquest and said they may be closer to finding out who is responsible for Mr Johnson's death.

Steve Johnson looks over the cliffs at North Head in Manly, where his brother Scott Johnson plunged to his death. ( Lateline: Emma Alberici )

"The inquest produced many leads that should still be pursued," he said.

"The killers probably still live among us.

"Now it's time for a homicide investigation… and there's lots to go on."

Steve Johnson made a plea for those responsible to come forward and help in the "progress being made toward humanity and healing in Australia".

He said their parents, who have died within the past five years, would have been pleased with the inquest's finding.

The coroner said it was almost certain Scott Johnson went to North Head to engage in sexual activity. ( Supplied: Johnson family )

US private investigator and journalist Daniel Glick, who Steve Johnson hired to investigate his brother's death, expressed his anger at the initial evidence from police that the area where Mr Johnson died was not a gay beat.

"It was so clear to everyone who lived in Manly and the northern beaches and indeed the gay community that this place was a beat," he said.

He believes there could be "many more" unsolved murder mysteries which were a result of gay hate crimes in past decades.

Mr Barnes offered his condolences to the Johnson family, who have long argued against the theory that Mr Johnson took his own life.

"When Scott Johnson's body smashed into the rocks … a life full of promise and exciting potential was tragically cut short and his family lost a cherished member," he said.

This is the third inquest to be held into the 30-year-old mystery.

The first inquest found Mr Johnson took his own life while the second did not make findings on how he fell.

Mr Barnes today admitted that initial investigations by police "regrettably" jumped to conclusions without examining all the facts.

"I'm sure that was not due to any malice or other improper motives, rather it was just easier to accept what seemed to be the most obvious explanation."

A spokesperson for NSW Police said the Homicide Squad's Strike Force Macnamir presented all available evidence to the Coroner gathered during its four-year investigation.

"The case will remain open and any new information provided to police relating to the circumstances of Mr Johnson's death will be thoroughly investigated," the spokesperson said.