Former Canberran Stephen Jelfs was 21 years old when he disappeared from Paddington in 1978.

He was 16 when he moved to Sydney to set up a florist and plant store in the central Sydney suburb.

A 2009 coronial inquest found Mr Jelfs had died but the cause of his death was unknown.

The reward of up to $100,000 is available for information that leads to the conviction of a person or people responsible for the death of Mr Jelfs, the NSW Homicide Squad's unsolved homicide team announced on Wednesday.

Detective chief inspector John Lehmann said Mr Jelfs lived in a Sutherland Street apartment with his partner, who last saw him at their home on April 24.

"He was a keen traveller, having travelled to Asia, Europe, the United Kingdom and the United States," he said.

"He was also a member of Sydney's gay community."

Police have been told Mr Jelfs was acting in a nervous and agitated manner in the months prior to his disappearance, which was out of character.

Police were also told Mr Jelfs had been associating with a person believed to be involved in the supply of prohibited drugs.

Extensive police inquiries have failed to reveal the circumstances surrounding Mr Jelfs' disappearance and death, including any suspicious circumstances.

Police and Mr Jelfs' family hoped the reward would be an incentive for someone to come forward.

His father, Neville Jelfs, was among family at the announcement in Sydney on Wednesday.

"I hope the reward will bring someone or something out to solve this matter, which has haunted my family and myself for 38 years," he said.

Police have urged anyone with information about Mr Jelfs' disappearance to call Crime Stoppers on 1800 333 000 or via nsw.crimestoppers.com.au

Information can be provided anonymously.