A former SAS corporal has told the Federal Court he watched Ben Roberts-Smith open fire on an Afghan prisoner before asking a comrade if they'd just witnessed an execution.
The witness codenamed Person 24 spoke on Monday about an April 2009 mission to a compound dubbed Whiskey 108 in Uruzgan province.
The witness said he watched Mr Roberts-Smith walk out of the compound holding a prisoner in his hand parallel to the ground.
"It appeared he had come off his feet," he said, either held by his pants or the back of his shirt.
He said the Victoria Cross recipient marched out of the compound and dropped the man on the ground before immediately opening fire "with a machine gun burst into his back".
After Mr Roberts-Smith stopped firing due to a weapon "stoppage" he walked back into a building and Person 24 said he turned to another soldier codenamed Person 14.
"Did we just witness an execution?" he recalled saying to Person 14.
Mr Roberts-Smith has strenuously denied the allegation.
The 43-year-old is suing The Age, The Sydney Morning Herald and The Canberra Times for defamation over reports that he committed war crimes and murders in Afghanistan between 2006 and 2012.
Person 24 said he watched Mr Roberts-Smith unload eight to 10 rounds of ammunition on the prisoner.
The incident was "right in my field of view," he said, adding he could not have missed it.
In February Person 14 testified that he noticed three Australian soldiers and a black figure outside the same Taliban compound.
"(The figure) was thrown to the ground and a thud happened like when a person hits the ground - that kind of noise of expulsion of air when a person is winded," Person 14 said.
A soldier raised their Minimi and fired an extended burst, Person 14 said.
Mr Roberts-Smith testified that he killed an insurgent within the rules of engagement.
On Monday Person 24 said he thought "it's extremely unfair how this thing has panned out" for Mr Roberts-Smith.
"I don't agree with (Mr Roberts-Smith) being here, under an extreme amount of duress for killing bad dudes who we went over there to kill," he said.
But he he was in court giving evidence in support of a comrade - Person Four - who was "adversely affected through blooding".
Before the team set out on the mission, Person 24 recalled sitting with his squadmates at Australia's base in Tarin Kowt when a patrol commander dubbed Person Five came into view.
"He was in a jovial manner, dancing a bit of a jig at our doorway and he said that 'we are going to blood the rookie'."
The phrase references giving a junior soldier their first kill in action.
The newspapers who seek to rely on a defence of truth, allege Mr Roberts-Smith stood by while the young trooper codenamed Person Four was ordered to execute one of two prisoners found in a tunnel.
Person 24 said before he watched Mr Roberts-Smith execute the Afghan male he heard Person Five screaming at Person Four to be with him inside the compound.
Before the troop extracted to their base Person 24 said another soldier tucked the prosthetic leg found on a dead prisoner's body into his backpack.
He confirmed he had drunk from the fake leg labelled Das Boot, many times.
Mr Roberts-Smith's barrister, Arthur Moses SC, accused the witness of giving false evidence.
"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.
He also denied talking to the media, saying he didn't agree with other soldiers doing so.
The trial continues.
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