Australia on Thursday announced a new investigative agency to build criminal cases against Australian special forces suspected of committing war crimes in Afghanistan.

The Office of the Special Investigator is to be formed after a four-year investigation into allegations and rumours surrounding behaviour of some soldiers in Special Air Service and Commando Regiments in Afghanistan from 2005 and 2016.

Benjamin Roberts-Smith, Australia’s most highly-decorated member of the armed services when he left the SAS in 2013, has been accused of by former colleagues of unlawful treatment of prisoners including illegally killing prisoners. The former corporal, who was awarded the Victorian Cross and the Medal for Gallantry for his service in Afghanistan, has denied any misconduct.

Defence Force Chief General Angus Campbell will make public a redacted report on the four-year investigation next week. 

Australian Prime Minister Scott Morrison said the new agency, headed by a retired judge or senior criminal lawyer, was needed because the workload would “seriously overwhelm” existing police resources.

“This report will be difficult news and all of our partners must be assured and those around the world who rightly hold the Australian Defence Forces in high regard,” Morrison told reporters.

“In Australia, we deal with this stuff and we deal with it honestly, but in accordance through the rule of law and by following the justice practices and principles that makes Australia what it is,” he added.

Two Australian Broadcasting Corporation journalists until recently faced potential prison sentences for using leaked classified defence documents as a basis for a 2017 report that detailed allegations of Australian soldiers killing unarmed men and children.

Police raided ABC Sydney headquarters with search warrants last year but prosecutors decided that charging the journalists would not be in the public interest.

Prosecuting alleged Australian war criminals is expected to take years.

“These are incredibly complex events involving actions and conduct in another country, in a war,” Morrison said.

“This is not a simple matter ... and so it will take as long as it needs to take to ensure we deal with our dual objectives of addressing the justice that is necessary in accordance with our laws and systems but also ensuring the integrity of our defence forces on which we all rely,” he added.

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Neil James, chief executive of the Australian Defence Association think tank, said the Australian military wanted solders to have their day in court to end a “continuous rumour mill.”

“It’s certainly the case that they’re going to take a long time because we’re talking about a complex investigation, witnesses will have to be interviewed under caution, in many cases those witnesses will be overseas, and in some cases it’ll be hard to interview them because they are in enemy-held territory in a war zone,” James said.

“So the complexity of this is going to be difficult, but it’s got to be faced because ... Australia has to face up to this, that things did go wrong and we need to fix it,” he added.

Around 39,000 Australians served in Afghanistan and 41 were killed.