Seem to remember a discussion at the Prime Minister's Cabinet Security Committee some years back - I think it was about the CHOGM Conference on the Sunshine Coast.
Lot's of back and forward talk about exclusion zones, maintaining CAPs, and who could legally authorise the shooting down of civilian aircraft over Australian territory.
Couple of points came from that.
Government realised that the laws didn't allow the Australian Defence Forces to operate live military engagements in Australia - let alone shoot - something to do with Constitution, and external powers, etc. The government had to chage the law to allow the military to act on that and subsequent occassions.
The other interesting debate centred around 'command and control', and collateral damage.
The argument went that if an F18 tried to shoot down a C172 and missed, the trajectory of the missile would be downwards, and might actually cause more damage on the ground at impact than the so-called terrorist' in the C172 could have managed.
Army argued that it would be better to have soldiers with shoulder launched surface to air missiles used - the argument being that if the SAM missed, it would fall [relatively] harmlessly to earth.
Air Force then argued that the Army command and control lines would preclude rapid response - i.e., soldier on the ground cannot act unilaterally - needs to have a line of authorisation up the chain of command, and back down again [even if there were pre-determined rules of engagement] - and it might take valuable minutes [or longer] to get an authorisation to fire.
Air Force argued that in the case of the F18 option, the pilot was a 'trained officer in Her Majesty's service' who had completed substantial training in the etiquette of instant decision making, and could be trusted to act immediately and correctly on pre-existing rules of engagement.
It was interesting to note, though, that Air Force decided against a full time CAP, and opted to 'scramble' an F18 from forward base Amberley if an 'incursion' occured - requiring the NOTAMing of a vast swathe of airspace as 'restricted' to allow enough time for the jet to get on station!