The witness codenamed Person 56 began his evidence in the Federal Court on Monday but stopped short during questions about a November 2012 mission to Fasil in Afghanistan.

His lawyer Sean Richter successfully argued the offence to which he would self-incriminate was of "the most serious nature".

"It's not something to be trifled with."

It is alleged that Person 56 and the war veteran executed two prisoners in Fasil, according to a statement documented in a previous ruling by Justice Anthony Besanko.

The former SAS corporal denies this allegation and all others The Age, The Sydney Morning Herald and The Canberra Times are defending as true.

The 43-year-old is suing the mastheads over reports claiming he committed war crimes in Afghanistan including murder, and acts of bullying and domestic violence.

Despite an offer of immunity from Justice Besanko Person 56 said he was still unwilling to speak and the newspapers' barrister, Nicholas Owens SC, did not seek to compel this evidence.

Arthur Moses SC, representing the Victoria Cross recipient, said a "deal" had been struck between the defence lawyers and Person 56 to pressure him into coming to court.

"It's true is it not ... that you felt threatened that if you didn't cooperate with them then they would call you and make assertions against you in relation to an allegation in Fasil involving yourself?" Mr Moses said

"Yes," Person 56 replied.

The former elite soldier said he finally agreed to speak to the newspapers' lawyers to get them "off his back," believing he would only be asked about a Darwan mission and not about Fasil.

If Person 56 did not speak in court about Darwan, he said thought he would be legally forced as a "hostile witness" and asked questions about the alleged execution.

The former elite soldier said during negotiations between his lawyers and those for the defence he was suffering from mental health issues and his wife was undergoing cancer treatment.

Asked about his understanding of a "throwdown," he responded it was carrying an item that can be used to make a situation look a certain way.

But when asked if he or any other member of Mr Roberts-Smith's patrol carried throwdowns in 2012, the witness successfully objected again on the grounds he might self-incriminate.

Another SAS soldier earlier told the trial he recalled Mr Robert-Smith discussing how throwdown weapons could be placed on a corpse for photographic evidence "so they could be deemed an enemy combatant".

As a trooper, Person 56 witness was deployed to Darwan in Afghanistan in September 2012 under patrol commander Mr Roberts-Smith.

Following the mission, his squadmates spoke about an incident whereby "an individual had been kicked off a cliff and subsequently shot".

It is alleged this incident - that Person 56 did not witness - involved Mr Roberts-Smith kicking an unarmed Afghan prisoner down a creek bed who was then executed.

Under cross-examination, he agreed his recollection of the conversation may be wrong, or have been influenced by other comrades, or by media articles.

Meanwhile, Mr Roberts-Smith has applied for another judge to hear an application to appeal his failed court bid in having his ex-wife quizzed about confidential information in an email account.

In January Justice Robert Bromwich said Mr Roberts-Smith had not provided sufficient bases to examine Emma Roberts about information obtained from her ex-husband's "RS Group Australia" email account.

He ruled that Ms Roberts accessed the email account for a "legitimate purpose" after suspecting that Mr Roberts-Smith was having an affair with a female solicitor acting for him in court proceedings.

Another judge will hear this application to appeal the ruling on April 20.

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