Police are offering a $250k reward to solve the infamous 'septic tank' murder case.

There are still no suspects in the murder of Brisbane man Wayne Youngkin 30 years ago but Queensland Police are hoping a $250,000 reward will help them crack the case.

Mr Youngkin was last seen alive in 1986 and his body was found in a septic tank last year at the Brisbane home where he lived with his uncle and grandmother.

The reward was announced on Monday and Police Minister Mark Ryan says indemnity from prosecution could also be offered to the murderer’s associates if they weren’t involved in the crime.

Detective Inspector Tim Trezise says a number of persons of interest have been identified but police have no suspects.

It’s possible more than one person was involved, he says.

“As to how Mr Youngkin got into that septic tank remains a mystery,” he said.

“It would probably be difficult if he was murdered in a different location. It would probably take a second person at least to transport him to the septic tank but we don’t know that to be the case.”

Police have not released Mr Youngkin’s cause of death but said he suffered “significant trauma”.

A car seat cover found with Youngkin's body. Image: QLD Police.

Det Insp Trezise said information from the community was crucial in cold cases because forensic evidence was usually scarce and many of Mr Youngkin's associates from that period were dead.

"It's very difficult for forensic evidence going back from that period to be the clincher in jobs like this," he said.

"It can be, but it's more commonly the case that people come forward with intimate knowledge of what happened."

He said forensic examinations were still being carried out in Australia and overseas on a brown striped seat cover found in the septic tank.

Earlier this year, police searched the Deagon Wetlands at Brisbane after receiving information that evidence could have been disposed of in the water.

However, the search was fruitless.

Late last year, investigators also urged a mystery man who rang Sandgate Police Station in August 1992 with "quite specific" information about the case to contact police again.