The NSW Government has offered a $100,000 reward for information that determines how a man died in Sydney's north more than 24 years ago.

The body of 27-year-old Scott Johnson was found at the base of Manly's North Head on December 10, 1988.

An investigation found no suggestion of foul play, and an inquest in 1989 determined Scott had committed suicide.

A second inquest last year returned an open finding, and referred the case to State Crime Command's Unsolved Homicide Team.

Detectives have now formed Strike Force Macnamir to review the case, which was featured in ABC TV's Australian Story on Monday.

They hope the $100,000 reward will encourage people to come forward with information that will help solve the mystery of how Scott died.

"At this stage, it is not known whether Scott's death is a result of suicide, misadventure or murder," Detective Acting Superintendent Chris Olen said.

"With a lack of witnesses and physical evidence, this is a very challenging case.

"What we need is fresh information to help us solve this case and bring some closure to the Johnson family."

"We understand that police need to look at all the different scenarios, and support them in that effort.

"But we believe in our hearts that Scott was murdered in a hate crime because he was gay."

Scott, who was American, came to Australia to live with his partner and study mathematics.

He died on the day he should have been celebrating completion of his doctoral work.