On the day decorated Australian soldier Ben Roberts-Smith is accused of ordering the execution of an unarmed Afghan civilian, he killed a Taliban soldier armed with an AK-47 and detonators, the Federal Court has heard.

Key points: Ben Roberts-Smith is suing three newspapers for defamation over a series of 2018 articles

Ben Roberts-Smith is suing three newspapers for defamation over a series of 2018 articles The articles allege the former soldier ordered the killing of an unarmed man, which he denies

The articles allege the former soldier ordered the killing of an unarmed man, which he denies The defamation trial will run for six weeks in 2020

The Victoria Cross recipient is suing three former Fairfax newspapers — The Sydney Morning Herald, The Age and The Canberra Times — over a series of 2018 articles that he says defamed him.

Mr Roberts-Smith, a former Special Air Service Regiment (SAS) soldier, has said the articles are defamatory because they portray him as someone who "broke the moral and legal rules of military engagement" and committed or solicited murder.

The articles state Mr Roberts-Smith kicked unarmed Afghan civilian Ali Jan off a cliff and ordered a junior soldier to kill him.

Mr Roberts-Smith denies those events ever took place.

Bruce McClintock QC, acting for Mr Roberts-Smith, told the Federal Court in Adelaide on Wednesday "these allegations are extraordinarily grave".

"My client has the Victoria Cross, has served our country for 10 years in Afghanistan

"There are many things he did, as many frontline soldiers do, that will impact him for life, but now he's contacted by two journalists who accuse him of five murders."

Mr McClintock said Mr Roberts-Smith had outlined his version of what happened when the small group of SAS troopers were dropped by a helicopter near the village of Darwan, in Afghanistan's Uruzgan province, on September 11, 2009.

Australian soldiers fly over Uruzgan Province in Afghanistan on a mission in 2011. ( Supplied: Department of Defence )

He said Mr Roberts-Smith was with his patrol when they were dropped on the bank of the Helmand River during an operation to hunt down a rogue Afghan soldier called Hekmatullah, who had killed three Australians in a "surprise attack".

"With his second-in-command — 'Person 11' — they saw someone across the river, who they believed was armed," Mr McClintock said.

"My client took off his body armour and his pack and, with only his weapon and a medical kit, swam across the river onto the land, to the rocks, killing him.

"On him, he had an AK-47. The rounds of one of my client's weapon went through the AK-47. He also had detonators, which were significant because they had never seen that kind of detonator in Afghanistan before, presumably foreign made.

"My client took the body down to the river so it could be photographed. [He] took the AK-47 and the detonators … and with them, swam back across the river.

"He's accused of killing an unarmed man. So far as being unarmed, my client put on display the AK-47 in the patrol room at Tarinkot with the bullet hole still in it."

'Soldiers would drink from prosthetic leg', court told

He said Mr Roberts-Smith's version of events had been corroborated by "Person 11".

The articles also state that Mr Roberts-Smith brought a prosthetic leg from a dead Taliban soldier back to Australia to use as a novelty beer drinking trophy.

"My client did shoot a man with a prosthetic leg — he was a member of the Taliban, probably lost his leg fighting the Russians. My client did not souvenir his leg though," Mr McClintock told the court on Wednesday.

He said the newspapers had disclosed a photograph of another patrol leader with the leg strapped to his backpack, souveniring it, which had "nothing to do" with Mr Roberts-Smith.

"My client will give evidence that there was one member of the SAS who never drank from that thing and that was him," he said.

"Why? Because he thought it was disgusting to souvenir a body part, albeit an artificial one from someone who had been killed in action."

Mr McClintock is seeking to access notes compiled by the journalists of conversations they had with their confidential sources before the articles were published.

But Sandy Dawson SC, acting for newspapers, told the court that lawyers for the former soldier were undertaking "classic fishing".

Victoria Cross recipient Ben Roberts-Smith (centre) with Keith Payne VC AM and Harry Smith SG MC. ( Supplied )

"There's only a truth defence in this case," he said. "The information the journalists had in their possession before the articles were published is irrelevant because they don't go to truth.

"The stakes are high for both sides, there's no doubt about that."

The articles were written by journalists Nick McKenzie, Chris Masters and David Wroe, who are also being pursued by Mr Roberts-Smith.

The Federal Court defamation trial is set to run over six weeks in June 2020.