Witnesses who gave evidence against Ben Roberts-Smith in his defamation case have been labelled liars, perjurers and gossips during a closing address by the war veteran's counsel.

The long-running trial is in its final stage, where barrister Arthur Moses SC is making closing submissions to the Federal Court in Sydney.

He took aim at the truth defence of publisher Nine Entertainment, which is being sued over newspaper stories published in 2018, and said its case was built on imprecise testimony, contradictory evidence, conjecture and speculation.

Mr Roberts-Smith is suing over what he says are false allegations published in the stories, including that he committed war crimes in Afghanistan, was a bully to SAS colleagues, and was a perpetrator of domestic violence. 

An SAS witness called by Nine, Person 14, previously claimed in court he witnessed Mr Roberts-Smith direct, via an interpreter, an Afghan soldier to execute an unarmed local man during a 2012 mission.

Another witness, Person 7, made allegations Mr Roberts-Smith used unnecessary force on Afghan civilians and claimed to have once heard him speak about a desire to "choke a man to death with my bare hands".

Mr Moses said the two were "plain and simple liars and perjurers". 

"Person 14 repeatedly lied to Your Honour over at least 15 pages of his evidence. The lies were dripping from the pages," Mr Moses said.

That was, Mr Moses said, until the notes of Chris Masters were produced, detailing what Person 14 had told the journalist, which prompted his lies to unravel.

Mr Moses dismissed Person 7's testimony as "quite frankly embarrassing".

"Person 7 is a man possessed and obsessed with Mr Roberts-Smith's Victoria Cross, to the point of his partner telling him to stop talking about it, on his own admission," Mr Moses said.

He said Person 7 was not an eye witness, but "a gossip".

"He would even make Mrs Mangel from Neighbours blush, in respect of his amount of gossiping concerning Mr Roberts-Smith," he said.

Another witness, Person 24, previously told the court he saw Mr Roberts-Smith execute an Afghan man with a machine gun during the 2009 raid of a Taliban compound known as Whiskey 108.

"That man is a liar," Mr Moses said, pointing to the "inconsistent versions" the soldier had previously supplied.

"That witness subscribed to the unjudicial dictum of 'it's not a lie if you believe it', with apologies to George Costanza from Seinfeld," Mr Moses said.

Mr Moses told Justice Anthony Besanko that Nine had not established any of the "grave allegations" it propounded, and said the "vice" of its truth defence was its opacity and lack of precision.

Mr Roberts-Smith was accused in Nine's defence case of committing or being complicit in six murders in Afghanistan, but Mr Moses said the details of some of them had evolved over time.

"It has been truly a shifting sands approach to how they allege certain murders occurred, who was involved, or why they occurred," he said.

"A sliding factual substratum which they have impermissibly sought to adapt to whatever argument might advance their case, in a desperate bid to justify their unjustifiable publications."

The closing submissions are expected to last until Wednesday.