The Office of the Special Investigator has been tasked with examining allegations of war crimes committed by Australian special forces soldiers in Afghanistan.

Last year's Brereton report found credible evidence of war crimes, referring to 36 matters made up of 23 incidents, including allegations of 39 murders.

The office's director-general Chris Moraitis confirmed the number of matters being investigated has since risen to be in the 40s.

Mr Moraitis indicated it could go higher.

"To be fair, I don't want to be held to that figure, because as we investigate, you have lucky days sometimes," he told a Senate estimates hearing on Tuesday.

Mr Moraitis said he was optimistic his office would be able to hand over an evidence brief to prosecutors in the first half of 2023.

He said he was "loudly confident" there would be enough evidence in the brief for prosecutors to prepare a case.

But Mr Moraitis refused to confirm whether any partial briefs had already been handed over, saying the investigation was still being finalised and he didn't want that information on the public record.

He added the investigation had been stonewalled by officials not being able to enter the country.

The Taliban retook control of the capital Kabul in mid-2021, hindering access to the nation.

But Australian officials have travelled to third countries to gather evidence.

Mr Moraitis said it was easier for United Nations personnel and journalists to gather some information about the alleged crimes if they have a presence in Afghanistan.

"It's one thing to go there and to have a conversation with a few people in Kabul," he said.

"It's another matter to go out into, let's say, a province, obtain evidence and then have a state-to-state relationship whereby you can use that evidence in an Australian court of law."

The number of staff in the office doubled last year, going up from 52 to 125 working across its three offices in Canberra, Sydney and Melbourne as well as AFP offices around the country.

Australian Associated Press