An elite soldier has told a Sydney court he witnessed war veteran Ben Roberts-Smith shoot dead an Afghan man during a 2009 mission and order a colleague to execute a second man.

Mr Roberts-Smith's high-stakes defamation trial against three newspapers and three journalists resumed in the Federal Court in Sydney today, after months of delays caused by COVID-19.

The Victoria Cross recipient denies allegations contained in stories published in 2018, including of unlawful killings overseas, bullying of colleagues and domestic violence. 

Publisher Nine Entertainment on Wednesday called a current Special Air Service Regiment (SAS) member who has been allocated the pseudonym "Person 41".

One of his four overseas deployments, to Afghanistan, included a 2009 Easter Sunday mission in which several Australian patrols were clearing a compound, codenamed "Whiskey 108".

Person 41 recalled there was "quite a lot of rubble around" after the compound was bombed and said troops found a hidden tunnel entrance while they were clearing a courtyard area.

The witness said he went into another room where he discovered a battery, wires and a "black sticky substance", which turned out to be opium, and decided improvised explosive devices were being made there.

He returned to the courtyard after hearing "commotion".

The court heard Mr Roberts-Smith was also in the courtyard, along with another soldier, Person 4. An older Afghan man wearing traditional clothing was "squatting down" nearby.

The witness said Person 4 asked to borrow his gun's suppressor, which Person 4 then began fitting to his own weapon.

"I then thought to myself, 'I think I know what's about to happen here,'" Person 41 told the court.

He said Mr Roberts-Smith grabbed the Afghan man by the shirt and picked him up before kicking him.

Person 41 said he stepped into a room, not wishing to witness "what was about to happen", and waited while a gunshot rang out.

When he returned to the courtyard, he saw the body of the man with a head wound and said Person 4 "seemed to be in a bit of shock".

The allegation about the older man's death was put to Mr Roberts-Smith during his evidence last year. 

The veteran said it was "completely false" to say that he instructed Person 4 to shoot the man, or that a separate colleague had already ordered Person 4 to do so before him.

Mr Roberts-Smith further denied that there were any men found in the compound's secret tunnel.

Person 41 later told the court he left the compound and was about to make his way up to two other buildings when he noticed Mr Roberts-Smith "frogmarch" another Afghan man outside.

He said Mr Roberts-Smith "threw" the man on the ground, turned him onto his stomach and then fired "three to five" rounds of his machine gun into the man's back.

"He (Mr Roberts-Smith) said 'are we all good, all cool', and I just said 'yeah mate, no worries'," he told the court.

The witness said he denied knowing what happened to "two blokes pulled out of the tunnel" when later asked by another colleague.

"I just wanted to keep quiet about the whole thing," he said.

"I was toeing the line, so to speak, I was a new trooper.

"It's the unwritten rule, you just go along with whatever happens."

The court has previously heard Mr Roberts-Smith deny allegations he carried a man with a prosthetic leg outside the Whiskey 108 compound, threw him on the ground, and shot him with a light machine gun.

Mr Roberts-Smith insisted he shot a suspected enemy who ran around a corner of the compound carrying a weapon as he went outside, and that an unnamed colleague shot a second enemy.

Earlier today, Justice Anthony Besanko explained why the evidence of Special Operations Command witnesses would not be accessible to the public as a real-time live stream.

The court will instead release a video of the session afterwards, to avoid the risk of "inadvertent disclosure" of sensitive information.

Person 41's identity is being kept so secret that a video feed of proceedings being viewed by the media is not including any view of him.

The trial continues.