The agency carrying out the probe also underwent a refurb to better protect the classified information it was handling.

The new details are contained in the Office of the Inspector-General of the Australian Defence Force's 2017-18 annual report, tabled out of sessions on Wednesday.

The inspector-general began investigating allegations of possible breaches of the Laws of Armed Conflict by members of Australian special forces soldiers in 2016.

Confidential documents leaked to the ABC in 2017 revealed allegations of unlawful killings, including the shooting and cover-up of a young boy in Afghanistan's Kandahar Province in 2012.

Those documents were the trigger for the controversial ABC raids in June, which sparked headlines worldwide and a political firestorm over press freedom.

Whistleblower, former military lawyer David McBride is currently facing charges in the ACT Supreme Court over the leaks.

Inspector-General of the Australian Defence Force, James Gaynor said the investigation gained "significant momentum" after a public callout for information in late 2017, through which additional lines of inquiry emerged.

More than 220 witnesses have been interviewed in connection with the investigation.

"At this stage it is not possible to predict how long it will be before the Afghanistan inquiry delivers its report, as this will depend on the number of additional lines of inquiry identified as the inquiry progresses," Mr Gaynor said.

Mr Gaynor stressed it was an administrative process, not a criminal investigation.

"This process is intended not only to ascertain whether there has been misconduct, but equally to exonerate those who may be affected by unsubstantiated rumours and allegations," Mr Gaynor wrote.

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As with a royal commission, the inquiry has the power to compel evidence.

It is being led by a reserve officer of major-general rank and includes one brigadier, three colonels (two from the Navy and one from the Army) and two lieutenant colonels (one Navy and one Air Force), also from the reserves.

At the time of the report, the inquiry was at phase three, where the team was examining each of the rumours and alleged incidents to establish their veracity.

"The inquiry is being conducted in private, because it has implications for operational security and persons with protected identities are involved," Mr Gaynor said.

"Other reasons for private proceedings are to protect the confidentiality of witnesses and to protect the reputations of persons whose reputations might unfairly be affected by unsubstantiated rumours."

Anyone who may be subject to adverse findings has been given a right of reply during this stage, he said.

The inquiry also necessitated an office fit-out, Mr Gaynor claimed.

"During the reporting period, refurbishment works were carried out within [the office] to enhance information security in connection with the inquiry," Mr Gaynor wrote.

"Given the range of statutory inquiry functions performed, and the prospect that [the inspector-general] will be required to undertake similarly sensitive inquiry and review work in future, it was considered that the modifications required to facilities would have longer term benefit."

Phase four of the investigation was to look at the cultural, psychological, operational and organisational factors that may have led to the alleged crimes, and the proliferation of rumours about them.

The fifth and final phase would be the report, of which some preliminary aspects were already in train.

"At this stage, it is not possible to predict precisely how long it will be before the Afghanistan inquiry delivers its report, as this will depend on the number of additional lines of inquiry that emerge as the inquiry progresses," Mr Gaynor said.

Meanwhile, the office is under growing pressure because of the number of increasingly complex and sensitive matters it is being asked to look at.

At the time of the report, it had 60 inquiries on the go.

The office also received nearly 400 new applications for redress of grievance (although 8 per cent fewer than the year before).

While it is still able to function within its set budget and existing staff, Mr Gaynor warned it was unsustainable in the long run.