Australia's top military officer insists he maintains full trust in the special forces following a torrid fortnight for his troops.
Asked whether he was concerned there were rogue elements within the force, Air Chief Marshal Binskin insisted he maintained confidence in his troops.
"I trust the special forces 100 per cent," he said.
Chief of Defence Force Mark Binskin backed his soldiers while savaging the actions of a person who sent a threatening letter to members of the SAS.
The letter related to the soldier giving evidence to an inquiry into the conduct of Australian forces in Afghanistan.
The Defence Department has asked the Australian Federal Police to investigate.
"I think it's absolutely disgusting that a disaffected person thought they could threaten a witness and look to influence the inquiry," Air Chief Marshal Mark Binskin told reporters in Sydney on Friday.
"That's a criminal act, and it will be handled appropriately."
Defence Minister Marise Payne was more circumspect.
"Any action of that nature is of course concerning, and I will let the federal police investigation take its course," Senator Payne said.
"It's an isolated incident as far as we're aware, but I don't intend to comment on that any further."
The sweeping investigation into Australia's elite troops in Afghanistan is looking into allegations of unlawful killings and disregard for human life.
The Inspector-General of the Australian Defence Force inquiry does not reveal the identity of witnesses and hearings are conducted in private.
"Witnesses are protected by law from intimidation and from liability for what they tell the inquiry," a Defence spokeswoman said.
The ADF has copped heavy criticism this week, with Malcolm Turnbull slamming as "utterly unacceptable" a photograph of Australian soldiers flying a Nazi swastika flag from their vehicle in Afghanistan in August 2007.
