He served a jail sentence for insider trading before his 30th birthday and is the star witness in the criminal trial of his former best friend, PR queen Roxy Jacenko's husband Oliver Curtis.

On Wednesday, John Hartman told the NSW Supreme Court he and Mr Curtis were "swept up in a fake world" and hatched an illegal plan in their early 20s to profit from confidential information on the sharemarket.

"It was almost a bit of a game in a sense. It wasn't real," Mr Hartman said.

"What we were doing was effectively front running or insider trading."

The sort of money that could be made in a day was "crazy, it's sick", he said, recalling a $100,000 windfall.

It was the "easiest money we could possibly have made" and the pair wouldn't consider any trades that would make them less than $10,000 or $20,000 in one hit, Mr Hartman said.

But the 30-year-old told the court he had made a "huge mistake" and had served 15 months behind bars for insider trading offences, some of which were linked to trades at the heart of his alleged deal with Mr Curtis.

Mr Curtis, supported in court by his wife and Sweaty Betty PR agency founder Ms Jacenko, has pleaded not guilty to the charges against him.

Mr Hartman gave a damning account of the pair's alleged plan to use information he acquired during his work as an equities dealer at boutique investment firm Orion Asset Management. He said both he and Mr Curtis had a gambling problem at the time.

"Unfortunately we came up with a plan whereby I would tell Ollie the stocks Orion would be buying ... so he could use that information," he said.

"The purpose was to make money out of the information I held. I controlled when Orion bought or sold stocks. I made a huge mistake by taking advantage of that opportunity."

Mr Hartman said the pair agreed to buy and sell contracts for difference, which can allow traders to profit from shifts in share prices, and "share the profits 50-50".

The court has heard the trades netted the men $1.43 million in profits between May 2007 and June 2008, some of which was spent on a $60,000 Mini Cooper and a $20,000 Ducati motorbike for Mr Hartman.

The pair also went on a luxury holiday to Whistler and Las Vegas, including shouting their friends helijet transfers "as we will all be very hung over and will not enjoy a two-hour drive".

Mr Hartman said his friends "didn't know where the money was coming from", apart from Mr Curtis.

"They just thought I was being generous," he said.

Secret messaging method

Mr Hartman said the men had met in a bar in the Sydney CBD on one occasion and had discussed that they would both be "in a lot of shit" if their dealings came out in court.

He told the court he would communicate with Mr Curtis using a BlackBerry that his friend had bought for him.

Mr Curtis had shown him a method of communication known as "pinning", he said, which was "like SMSing somebody" but the information was not stored by telecommunication providers.

"He told me a story I can still remember 10 years later now," Mr Hartman said with a small laugh.

Mr Curtis had said the only way to access the content of the messages would be to "pull a satellite out of the sky", Mr Hartman claimed.

"We both understood we needed to be careful. He understood exactly what the information was, where it was coming from."

The court has heard the men grew up in Mosman on Sydney's lower north shore, attended the same high school and lived together in a luxury apartment in Bondi. Mr Hartman said they used some of the proceeds of their trading to pay the $150,000 rent on the property upfront, along with two plasma televisions.

"We were best friends," Mr Hartman said.

Emails between the pair, tendered in court, show Mr Curtis talking excitedly in February 2008 about Bondi being the "BEST place to live and Notts Avenue is THE street so you don't get much better".

In spite of their windfall profits they also had minor disagreements, including Mr Curtis asking Mr Hartman to "make a more conscious effort to lighten the use of electricity like turning off the lights etc!" to save on bills.

But the court has heard the relationship has since soured more dramatically.

Mr Curtis' barrister, Murugan Thangaraj​, SC, has asked the jury to consider if Mr Hartman had turned on a "good mate" to reduce his own jail time.

Mr Thangaraj told the jury on the second day of the trial that Mr Hartman's co-operation with authorities, including giving evidence in his friend's trial, had translated to "less time in jail".

Mr Hartman was released on a good behaviour bond in March 2012.

He agreed that the total profit from his personal trading was $5.8 million, of which only $1.59 million was seized by the corporate watchdog when his account was frozen. He was also allowed to keep his motorbike, which he later sold, and a BMW he bought when he traded in his Mini Cooper.

Mr Hartman told the court that his motivation at the time was "to make money".

"Whilst it's all relative, at the time I didn't have access to a large amount of money ... The opportunity to make a lot of money from taking advantage of the information was there."

Mr Hartman agreed Mr Curtis came from a "wealthy family" but denied he would have been able to give him the money he wanted.

The trial before Justice Lucy McCallum continues.