Paul Murray was last seen alive by a neighbour on the outskirts of Lightning Ridge in northwestern NSW on March 19, 1995.
He was reported missing one week later.
Mr Murray, then aged 40, owned an opal mining field about 8km from the town and lived in a camp at the site.
Officers searched his campsite but did not notice anything suspicious or unusual, while aerial and land searches could not find any sign of the man.
On April 22, two graziers found his naked, decomposing body in scrubland, about 2km from his campsite.
"It was quite a brutal and gruesome scene," Homicide Squad Commander Joe Doueihi told reporters in Dubbo on Wednesday.
A post-mortem 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, and in 2012, unsolved homicide unit detectives established a strike force to investigate further.
Authorities lifted a reward for information on Wednesday to $1 million.
More than three decades on, his family wants answers.
"A million dollars is a lot of money and if you're out there and you're not feeling too well these days or you have a problem in life and you know something, why not contact the local authorities?" his brother Simon Murray said.
"Not only would you be helping us as a family with answers, justice and closure, we could get on with our lives."
Police have previously attempted to follow a string of unusual circumstances surrounding Mr Murray's death.
He was found naked with a towel wrapped around him and burrs in his feet, suggesting he may have been running away or had left the campsite with some urgency, Det Supt Doueihi said.
Mr Murray had also bought a gun shortly before his death.
He was known for his generosity, often loaning money to friends and associates, and kept track of these dealings in a little black book that has never been recovered.
A watch found about 50m from his body has also been investigated as police cannot confirm it belonged to Mr Murray.
The opal miner was remembered for his infectious laugh and devotion.
Earlier in his life, he had been a competitive swimmer, a gymnast and a boxer.
When he was 16, he has hit by a speeding motorist, giving him an irreversible brain injury that affected his connectivity, walking and appearance and made him vulnerable, his sister Rosemary Pearse said.
"Paul only saw the best in everybody and believed what he heard was truthful," she said.
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