A key witness implicating a man in a Melbourne underworld murder is a killer himself who has got away with murder in exchange for his evidence, a jury has heard.
The prisoner alleges he helped the late Carl Williams organise the execution of Graham "The Munster" Kinniburgh, with the aid of accused hitmen Stephen Asling and the now-dead Terrence Blewitt.
Asling, 56, has pleaded not guilty to murdering Kinniburgh, who was shot dead outside his Kew home in December, 2003.
Under cross examination by the defence, the prisoner admitted he was party to a deal to give evidence against Asling to escape prosecution for another murder, for which he was paid $20,000.
The witness denied falsely implicating Asling, to get a possible earlier jail release.
"That never really occurred to me," he said.
"I was too busy writing statements."
The jury heard the prisoner had written to the Director of Public Prosecutions to let them know about some "explosive allegations".
"I told the truth," he said.
But he also admitted telling lies to authorities, including lawyers, courts and police over the years, to protect his self-interest.
The prisoner said neither he, nor Williams, had ever met Kinniburgh but knew of him as a member of the criminal underworld.
The prosecution has alleged four people - the prisoner, Williams, Asling and Blewitt - were involved in the joint criminal enterprise to kill "The Munster".
The prisoner told the jury Williams gave him a bag, which he in turn passed onto Asling, containing two guns, speed and about $20,000 to do the job.
But the jury heard the target was originally Kinniburgh's associate Lewis Moran.
It changed to Kinniburgh because William's plan to kill Moran, whom he hated passionately, was going nowhere, the jury heard.
The trial continues Monday.
© AAP 2017