Hillier was reported missing by a relative last week. His travelling companion, a 43-year-old man, has been charged with his murder.
Both were last known to have lived in Lake Haven on the NSW Central Coast.
The younger man had been reported missing about two years ago.
Detective Superintendent Des Bray said it was believed the pair left NSW about a month ago and were travelling in a motorhome Hillier had recently purchased.
On Monday, a call to the ambulance service indicated he had been injured after falling from a vehicle while drunk on the way back from a fishing trip.
But when crews arrived at the Venus Bay property, he was already dead.
Bray said subsequent investigations led police to believe Hillier had not been injured in a fall but had been killed.
He said the motive or the exact location of his death were not clear with police working to determine his movements since leaving NSW.
“His injuries are significant and we don’t believe it’s consistent with falling from a vehicle,” Bray said.
“He had been dead for a period of time.
“Our picture of what happened and how it happened is pretty incomplete at this stage.
“Except to say that we can clearly establish that Peter was the victim of a murder as opposed to some accidental death.”
The man charged with his murder was refused police bail and was expected to appear in Port Lincoln Magistrates Court on Tuesday.
Detectives also returned to the Venus Bay address on Tuesday and issued an appeal for anyone with dashcam footage from the Flinders Highway or surrounding roads on Sunday, May 29 to make contact.
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