Who's the enemy - Taliban or Afghan army?

The killing of three Diggers and wounding of another seven by an Afghan soldier on Saturday is troubling on several fronts.

Not only is it one of the biggest death tolls to have been suffered by Australian troops in one incident, but it casts a cloud over the entire Afghan mission.

The central task of Australian and NATO forces is to train local security forces so our troops can come home.

If the Afghan soldiers we train are not of an acceptable standard the country could plunge into chaos the moment we pull out.

This new attack is not an isolated one. In May, Lance-Cpl Andrew Gordon Jones was shot dead by an Afghan soldier and there have been a spate of similar incidents in which US, British and German troops have been killed by their allies.

Some of these killings have been committed by Taliban infiltrators, but the vast majority have been carried out by properly vetted ANA soldiers - usually the result of a brain snap following an argument or a cultural misunderstanding.

Australian troops operate hand-in-hand with the ANA. At the small forward bases that dot Oruzgan province, Diggers sleep within a few metres of ANA troops and carry out patrols with them.

The Australians have a good relationship with the Afghans, but often roll their eyes at their lack of discipline.

But while the Australians normally tolerate this with good humour, these latest killings will leave some wondering who they should be more worried about - the Taliban or the ANA.