I dont think we are under that great of threat from the air when it comes to a terrorist attack, as has been seen in the past, the terrorist weapon of choice is a car, van or backpack.
and as for security protecting us from such weapons? well there is none, in my previous life as a courier operator, the amount of access granted to van drivers with a logo on the side is amazing! the premiers department in syd was a regular client, no id, no check nothing, just drive on under the building, same for the International and domestic terminal at YSSY, i had regular deliveries of promotional material to the Sanity record stores, straight under the terminal, in a loaded van, no id, nothing, then carry 4 boxes full of 400 CD to the elevator, and into the "sterile" side of the airport, again, no xray, no id check, not even a glance from the security guards.
sadly in Sydney, it doesn't take much to stop the city. just look at the train breakdown the other night! 1 train, a vast majority of the city is thrown into chaos...
you can bring the entire metro area to a standstill with just 4 cars in a few strategic intersections, and just 2 trains stopped in certain places..
i think, if there will be an attack on Aussie soil it will come from a backpack or vehicle..