Sydney's most high-profile insider trading trial has pitted former school friends against each other as the defence barrister for Oliver Curtis, investment banker and husband of PR queen Roxy Jacenko, accused the star witness of turning on a "good mate" to reduce his own jail time.

Mr Curtis, 30, is charged with conspiring with his childhood friend, John Hartman, to commit insider trading offences that netted the pair an alleged $1.43 million between May 2007 and June 2008.

Mr Hartman, who has already served jail time for insider trading, is the prosecution's star witness.

"Is he honest? Is he reliable? Can we trust him?" Mr Curtis' defence barrister Murugan Thangaraj, SC, said in his opening address to the jury on Thursday.

The Crown has alleged the men, then in their early 20s, struck a deal to use confidential information to profit illegally from trades related to shifts in share prices. The pair had grown up together in Mosman on Sydney's lower north shore and attended the same high school.

"Without Mr Hartman's evidence there is no case," Mr Thangaraj said.

The NSW Supreme Court has heard the men spent some of the $1.43 million on lavish living expenses including half of the $300,000 rent for a Bondi pad, a $60,000 Mini Cooper, a $20,000 motorbike and trips to Whistler and Las Vegas.

"Mr Hartman made a deal with [authorities]," Mr Thangaraj said.

"He made an allegation against a good mate, Mr Curtis, and he had two benefits from that."

The first was "less time in jail" for his own insider trading offences, Mr Thangaraj said, and the second was "benefits of a deal he made" with the investigating body, the Australian Securities and Investments Commission (ASIC).

"How do we describe that deal? Was it an extraordinary deal?" Mr Thangaraj said.

Crown prosecutor David Staehli, SC, told the jury on Thursday that Mr Hartman was sentenced to a maximum of three years in prison for insider trading offences, some of which were linked to his alleged deal with Mr Curtis, and was released after 15 months.

He foreshadowed that the defence would raise questions about Mr Hartman's credibility, and the Crown would counter them.

"Was he acting in self-interest when he spoke to ASIC about Mr Curtis or was he coming clean on the truth?" Mr Thangaraj said of Mr Hartman.

Mr Thangaraj said there was no question the men carried out the trades of contracts for difference, which allow traders to profit from shifts in share prices.

But the question for the jury was whether the men "intentionally entered into an agreement" to use confidential information acquired by Mr Hartman in his position at boutique investment firm Orion Asset Management.

The Crown has alleged Mr Hartman "procured" or "encouraged" Mr Curtis, then employed at boutique investment banking and corporate advisory firm Transocean Group, to buy particular contracts for difference exploiting that insider information.

Mr Thangaraj told the jury that Mr Hartman was "caught" in mid-2009 by his broker, who made it clear he would tell Mr Hartman's boss about particular trades.

Mr Hartman then spoke to his lawyer, and later to ASIC.

Ms Jacenko, founder of the Sweaty Betty PR agency, appeared in court to support her husband on the second day of the trial.

The trial before Justice Lucy McCallum continues. Mr Hartman is expected to give evidence on Monday.