John Giannopoulos died from severe head and abdominal injuries, but investigators have been unable to establish a motive for the brutal inner-city killing.
The 72-year-old was found by two associates inside a unit on King Street, Newtown, about 10.30am on July 17, 2002.
A 2006 inquest found he had died from multiple injuries, inflicted by an unknown person or people, about one day before he was found.
Police say Mr Giannopoulos was by all accounts a very generous and well-liked member of the community, particularly among other Greek immigrants in the area.
The DNA of an unknown male was found on a glass in the unit and there was no sign of forced entry, leading police to believe he might have known his killer.
Homicide Squad commander Danny Doherty said the reason for Mr Giannopoulos's likely murder remained unclear, but someone in the community knew what happened to him.
"John, or Yanni as he was known also in the community, was a really well-liked, gregarious, generous and gentle man," he said on Thursday.
"For someone to brutally assault him and cause him significant injuries and kill him in his own home - it's hard to fathom."
Police released CCTV imagery of a woman who might have been in a relationship with Mr Giannopoulos as investigators encourage her to come forward.
The woman, described as being of Indian or subcontinental appearance, might have had a teenage daughter at the time, police said.
She was captured by cameras at a service station in Newtown on the day Mr Giannopoulos was found dead.
Detective Superintendent Doherty said police did not believe the woman had any involvement in the killing.
Mr Giannopoulos's niece, Vicky Vanzanella, joined detectives to appeal for information, saying the death had been tough on her family, especially her mother.
"She's not in the best health, we'd just like to give her some news," she said.
"Hopefully we can be happy that someone has been brought to justice for this cruel act."
Ms Vanzanella urged the woman who was possibly in a relationship with her uncle to help investigators however she could.
"If she did care about him, she should come forward and try and give us as much information as possible," she said.
The $1 million reward is being jointly offered by the NSW government and police.
A re-investigation of the case was launched following an unsolved homicide review in 2018.
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