Now, 37 years later, police are announcing a $100,000 reward for information about the cold case.

"It's about time," says his father, Neville Jelfs, 86, who joined police on Wednesday in welcoming the reward.

Stephen Jelfs was a 21-year-old gay florist - known to friends as "The Boppa" - when he disappeared. Among his clients was a Kings Cross criminal involved in drugs and the sex industry, his father told Fairfax Media.

"Steve used to provide plants for this guy and was in and out of his house from time to time," his father said.

When a notebook went missing from beside a telephone in the house, the criminal suspected the florist of stealing it.

But police had not seemed interested in this theory at the time, Mr Jelfs said.

While no trace of Stephen was ever found, a coronial inquest in 2009 found he had died but that the cause of death was unknown.

The investigation is now with the Homicide Squad's Unsolved Homicide Team.

Detective Chief Inspector John Lehmann, from the team, said the young man had been acting in a nervous and agitated manner in the months before he disappeared and that this was out of character for him.

Police had been advised that he was associating with a suspected drug supplier.

"Stephen lived in an apartment at Sutherland Street, Paddington, with his partner who last saw him at the address on 24 April 1978," Chief Inspector Lehmann said.

"He was a keen traveller, having travelled to Asia, Europe, the United Kingdom and the United States. He was also a member of Sydney's gay community."

Stephen, originally from Canberra, moved to Sydney at 16. He later ran a florist business, first a shop in Kings Cross and, at the time of his death, the Plant Gallery in Paddington.

Among his friends was Peter Rolfe, who is now the president of the advocacy group Support After Murder, and who campaigned for the reward.

Stephen was a popular figure and extremely knowledgeable about horticulture. Radio broadcaster John Laws was among his customers at the Kings Cross shop, Neville Jelfs said. He recalls that Laws would often say on air: "And how's The Boppa this morning?"

At the time of his son's disappearance, police had told him they suspected Stephen must have been involved in drugs because "he had so much money to throw around", Mr Jelfs said.

Mr Jelfs, who was in the air force at the time, never believed this. Among Stephen's friends was a wealthy man who owned multiple properties and Stephen had earned lucrative commissions for helping him sell them.

Mr Jelfs has travelled to Sydney from his home north of Newcastle for the reward announcement.

He said he did not know why police in 1978 had not pursued his information about the criminal.

"It was more than a suspicion," he said. "Steve named the guy – to me and one of his brothers."

Anyone with information has been urged to call Crime Stoppers on 1800 333 000 or to visit the Crime Stoppers online reporting page.

- with Megan Levy