A former elite soldier asked a comrade "did we just witness an execution" after seeing war veteran Ben Roberts-Smith allegedly shoot a man in the back outside a Taliban compound, a court has heard.

Mr Roberts-Smith is suing The Sydney Morning Herald, The Age and The Canberra Times, claiming stories published in 2018 defamed him by including false allegations of unlawful killings, bullying and domestic violence. 

A Special Air Service Regiment (SAS) witness codenamed Person 24, called by publisher Nine Entertainment, today told the Federal Court of an April 2009 mission at a compound nicknamed "Whiskey 108".

During that mission, Mr Roberts-Smith was accused of unlawfully killing an Afghan man with a prosthetic leg.

Person 24 said after the assault team entered Whiskey 108, he saw Mr Roberts-Smith walk out of an entrance. 

"He was holding a man in his hand, it appeared he'd come off his feet and he was being held parallel to the ground," Person 24 told the court in Sydney. 

"He marched approximately 15 metres, directly out from that entrance, dropped the man on the ground and immediately began with a machine gun burst into his back."

Person 24 said the man had been making a "grunting noise".

He told the court Mr Roberts-Smith fired "eight to 10" rounds before his weapon suffered a stoppage and he went back in the direction he'd come from.

The witness was with another SAS colleague, Person 14, at the time, the court heard.

"I recall saying to Person 14 at the time 'did we just witness an execution?'" Person 24 said.

Person 14 has previously told the court he saw "a black object which was similar to a human" being taken outside the compound and thrown to the ground with a "thud".

He said the object was shot but couldn't identify which soldier was shooting.

Mr Roberts-Smith last year told the court a man he killed that day was an armed insurgent coming around the corner of the compound. 

Under cross-examination by Mr Roberts-Smith's barrister, Arthur Moses SC, Person 24 denied his evidence was false.

"I wouldn't, in a court of law, give that evidence if it was false, I've just sworn an oath, Mr Moses," he replied.

Person 24 admitted under cross-examination that he didn't see the alleged shooter's face, which was also camouflaged. 

But he told the judge he could still tell it was Mr Roberts-Smith.

"I could tell from the way that he walks," he said.

"He has a hunched-over walking style when he's working. As soon as he walked out of that building, I could tell by the size, the shape, and his characteristic demeanour."

He also denied a suggestion from Mr Moses that Person 14 had suggested what he should tell the court.

The former soldier, who was medically discharged in 2017, agreed the mission was the subject of "much talk" over the years. 

"It has always been veiled, very veiled. It's something people didn't want to talk about," he said.

The witness denied speaking to the media about Whiskey 108.

He admitted he gave Person 14 a "welfare call" after Person 14's first day giving evidence to the court.

"What did he say to you?" Mr Moses asked.

"He said it was tough," the witness replied. 

"He mentioned you went at him like a rabid dog."

Person 24 earlier told the court he heard another comrade discuss "blooding the rookie".

"Person 5 came to the doorway of our patrol room and he was in a jovial manner dancing a bit of a jig ... and he said that we are going to blood the rookie," he said.

The witness said he understood that to refer to facilitating another soldier, Person 4, to record his first kill on a mission.

The trial, before Justice Anthony Besanko, continues.