Paul Murray was last seen alive on the outskirts of Lightning Ridge, an outback town in northwestern NSW, on March 19, 1995.
Mr Murray, then aged 40, owned an opal mining field about 8km northwest of the town and lived in a camp at the site.
A local resident had picked him up and dropped him off at the edge of town, becoming the last known person to see him alive.
Mr Murray was reported missing one week later.
After an extensive search, two graziers found his naked, decomposing body in scrubland on April 22, about 2km from his campsite.
A post-mortem examination found no obvious cause of death and no signs of trauma.
Further investigations and a coronial inquest in 1996 all failed to solve how or why he died.
In 2012, unsolved homicide unit detectives established a strike force to investigate further.
At the time, the NSW government offered a $100,000 reward for information, later increased to $500,000.
Authorities will again lift the reward on Wednesday, more than three decades on from Mr Murray's death, to $1 million.
Mr Murray's brother, Simon Murray, will join police in announcing the new reward on Wednesday morning.
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