Details of the alleged extortion attempt emerged in the NSW Supreme Court during cross-examination of a witness crucial to the Crown's case against Mr Medich.

A wealthy property developer, Mr Medich has pleaded not guilty to ordering the the 2009 contract murder of Michael McGurk, a former business partner who had tied him up in lawsuits.

The man who arranged the shooting, Lucky Gattellari, 66, has received a significant cut to his prison sentence for co-operating with police.

Gattellari has testified Mr Medich, 68, was the "big boss" who paid $500,000 for the job, eventually carried out by two other men.

But Gattellari is now facing fraud charges for allegedly attempting to elicit millions from Mr Medich with the help of Rogerson, McNamara and others.

Gattellari is yet to enter a plea but said the charges would never be proved.

He claimed that several years ago in prison, he and another inmate, Shayne Hatfield, conspired to catch Mr Medich trying to buy Gattellari's silence in court.

"I'm very anxious that we do something about trapping Ron," he wrote in his diary. "I want Medich to show how guilty he is. That will ease my mental stress I'm feeling thanks to this fucked system."

But Gattellari claims he never expected to receive any money as homicide investigators working on the Medich case knew of the plan and wanted to see how far it would progress.

He later said police never gave him express authority to carry out the plan but never told him not to, either.

Mr Medich's barrister, Winston Terracini, SC, grilled the witness on aspects of this plot and the reliability of his evidence.

"Did you ask Shayne Hatfield to engage the services of Roger Rogerson and Glen McNamara to assist you in defrauding Mr Medich?" he asked.

"After further discussions with police I did that yes," Gattellari responded. Rogerson had visited him in jail in April 2014, the court heard.

"Roger Rogerson was going to get an offer to the Medich camp that for a certain amount of money I could be got at to not give evidence," Gattellari said.

"The original amount we discussed was $10 million before the committal hearing and $15 million after the committal."

Mr Terracini asked Gattellari if he had devised another plot in which Rogerson and McNamara would extort money from Mr Medich with the false promise of arranging Gattellari's execution in jail.

Gattellari said that had only ever been "a joke", "a nonsense".

He called Rogerson and McNamara "2 f---wits" in a diary entry after the extortion efforts "hit a brick wall". He said the pair never knew police were aware of his plans.

The court heard that in 2013 he asked two police officers involved in the Medich case: "Is it a crime if I agree to do something but don't go through with it?"

Mr Terracini put it to Gattellari that he had been trying to implicate police in the plot, which the witness denied.

"You're a habitual liar," the barrister put to Gattellari, who rejected the characterisation.

Mr Terracini: "You don't hold yourself out to be an honest man, do you?"

Gattellari: "I'm as honest as the next man."

He is serving 7½  years in jail for the murder of Mr McGurk, 45, and the subsequent intimidation of his widow, Kimberley. Mr McGurk was shot in the back of the head in the driveway of his home in Cremorne on Sydney's north shore on September 3, 2009.

Gattellari's evidence continues.