"Even Dennis Denuto would know this," exclaimed the Sydney silk to the star witness in the criminal trial of Oliver Curtis, banker and husband of PR queen Roxy Jacenko.

It was an unlikely reference to the bumbling suburban solicitor in Australian film The Castle in a NSW Supreme Court trial bristling with experienced lawyers.

John Hartman, the former best friend of Mr Curtis, is the star witness in the prosecution's insider trading case against Mr Curtis and has been accused of giving evidence against him as part of a deal to reduce his own jail time for insider trading.

The prosecution alleges the two men, then aged in their early 20s, hatched an illegal plan to use confidential information acquired by Mr Hartman during his job as an equities dealer at boutique investment firm Orion Asset Management to cash in on the sharemarket.

Supported in court by Ms Jacenko, his wife and founder of the Sweaty Betty PR agency, Mr Curtis has pleaded not guilty to the charge.

On Thursday, the fourth day of the trial, the defence team ramped up its attack on Mr Hartman and suggested he "offered up" Mr Curtis to the authorities as a "bargaining chip".

Mr Hartman has already served 15 months behind bars for his own insider trading offences, and received a reduced sentence by agreeing to give evidence against Mr Curtis about trades at the heart of his alleged deal with his former friend. He was released in March 2012 on a good behaviour bond.

"You knew if you could offer someone up it would put you in a better position. Even Dennis Denuto would know this," said Murugan Thangaraj, SC, for Mr Curtis.

"I just told them what I knew," Mr Hartman said.

The court has heard Mr Hartman was "caught" in mid-2009 by his broker, who made it clear he would tell Mr Hartman's boss about his insider trading.

"I was desperate," Mr Hartman told the court. "I knew I'd been caught. I knew that the authorities would find out."

But he said he did not think about naming his friend to secure himself a "benefit" with authorities.

"They could have known about Ollie already," he said.

The court has heard allegations the men, who grew up together in Mosman on Sydney's lower north shore and were then aged in their early 20s, made $1.43 million between mid-2007 and 2008 by using inside information.

But Mr Hartman made millions more by himself. He agreed on Thursday that he reached a point where he decided he didn't "need" Mr Curtis for trades.

"You turned tens of thousands of dollars into millions of dollars in profits by yourself, didn't you?" Mr Thangaraj said.

"Yes, sir," said Mr Hartman.

The trial before Justice Lucy McCallum continues.