The naked and decomposed body of 40-year-old Paul Murray was found in scrub by two graziers about two kilometres from his camp site on April 22, 1995.
He owned an opal-mining claim about eight kilometres northwest of Lightning Ridge and lived in a camp at the site.
NSW Police said he was last seen alive on March 19, 1995 by a local, who had driven him to a location just outside town. His family reported him missing a week later.
A post-mortem examination found no signs of trauma or obvious cause of death and a 1996 inquest failed to determine how he died.
Following a review in 2012, the matter was referred to detectives from the State Crime Command's Unsolved Homicide Unit, who established Strike Force Huddleston to continue investigating his death.
Detective Chief Inspector David Laidlaw said police remain open-minded about the case.
"Paul's body was in an advanced state of decomposition and as such, subsequent investigations and an inquest failed to deliver a concrete answer as to what may have happened to him," he said on Thursday.
"Police have always kept an open mind as to the circumstances of his death and hope this reward can encourage the flow of new information."
Paul's sister, Rosemary Pearse, said her brother was a generous man.
"He would always offer to help anyone in need financially and only ask they repay him when their situation improved," she said.
"Any information, no matter how small, would assist police and may be what is needed to provide myself and my family with some answers after all these years.."
