Duncan Campbell, 58, was last seen at Coffs Harbour on the NSW north coast in February 2024 and was reported missing a few days later.
Eight days after his disappearance, his body was found more in bushland on the city's outskirts near the campsite where he was living.
While his death was initially deemed not suspicious, police said further medical evidence revealed Mr Campbell was the victim of a "savage and brutal" attack that ended his life.
Homicide Squad commander Danny Doherty said the reward offer was about finding answers for the father and grandfather's loved-ones.
"This was really an atrocious act of cruelty, but more than that this was a savage murder," he told reporters on Tuesday.
Mr Campbell was a decent man who was beloved by his family but had fallen on hard times, leading to him becoming homeless, Detective Superintendent Doherty said.
"This is a vulnerable man who wouldn't hurt a fly, who's found himself in really unfortunate circumstances," he added.
The 58-year-old's body was badly decomposed when found, which initially led police to believe there had been no foul play.
But later testing revealed he had sustained significant injuries from an assault that led to his death, police said.
Since then, investigators had extensively reviewed CCTV footage and conducted interviews with associates and community members before announcing the reward, which they hoped would trigger vital information from the public.
"We just need that little bit of a jigsaw piece to help us get that final bit of information that may be able to turn some persons of interest into suspects," Det Supt Doherty said.
"(The reward) $500,000 is a lot of money ... but you can't put a price on justice."
Mr Campbell's daughter Brittany expressed her grief at her father's loss.
"Living without dad, and without knowing what happened to him, has been incredibly painful for my siblings and myself," she said.
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