The Chief of the Defence Force, Angus Campbell, has conceded he was surprised by the speed of the Taliban takeover in Afghanistan last month.

The militant group swept into Kabul almost two weeks before the US military's set withdrawal date of August 31, shocking intelligence analysts who had predicted the Afghan national army would hold them off for several months at least. 

It forced a chaotic evacuation mission from Kabul by allied forces, leaving some unable to be rescued.

In early June, General Campbell also dismissed the prospect of a rapid Taliban takeover, telling a Senate estimates hearing: "I do not think that the situation is at all assured in terms of the Taliban's claimed ascendance."

"I think this is very much going to be a negotiated settlement," he said.

But this morning, General Campbell admitted his prediction was wrong and said he was surprised by the speed of the "cascade collapse" in Afghanistan.

"I wasn't anticipating ... that the collapse would be so immediate," he said.

"Collapse, [Taliban] accommodation or Afghan government success were all possibilities, but by the momentum of the Taliban effort the accommodation or cascade collapse were more likely.

"But I don't know of anyone who predicted — other than in the glory of 20/20 hindsight — how quickly it would occur."

'Interesting' deployment choices and bribery blamed for rapid collapse

General Campbell said the collapse was accelerated by the departure of Afghanistan's former president, Ashraf Ghani, who fled the country as Taliban fighters approached the capital, and some "interesting" force deployment choices.

It's not clear exactly what deployment "choices" General Campbell was referring to.

But yesterday the head of the Chief of the Defence Staff in the United Kingdom, General Sir Nick Carter, said that the Taliban likely bribed some Afghan soldiers not to fight them. 

"It was the pace of [the collapse] that surprised us, and I don't think we realised quite what the Taliban were up to," he told the BBC.

"They weren't really fighting for the cities they eventually captured, they were negotiating for them. And I think you'll find a lot of money changed hands as they managed to buy off those who might have fought."

The United States military was also fiercely criticised for leaving their hub at Bagram Airfield north of Kabul in early July without properly notifying the Afghanistan military.

General Campbell hailed Australia's massive evacuation airlift from Kabul, saying the 4,100 people flown to safety by the military was "way beyond the number which was initially anticipated".

"Not everyone who wanted to be withdrawn was able to get to the airport ... and that lift could not continue beyond August 31," he said.

"So the entrails of exactly how we did it will be pored over. But Australia should be proud of what we were able to achieve in a very very difficult situation."