That Hornet you speak of is for the Skyfire in Canberra - it's only here for an airshow.
@Bryce - The closes armed Air Force base is Williamtown roughly 5'ish hours via car. So they'd most likely scramble a Super Hornet/s which can do close to 2,000km/h - for safe keeping, let's say they travelled at 1,500km/h, distance is 500km (on a road - straight path flying would most likely be closer to 300km, even less) that would roughly take 15 minutes. We also have ships in Sydney with Anti-Air capabilities which could cruise along the east coast with radar following the movements of enemy aircraft - The Super Hornets would most likely go to the enemy jets as opposed the city. So, if they fled, they'd most likely be met halfway.
However, in saying this... enemy aircraft would most likely have to travel from the North of Australia which Tindal (Northern Territory) have 75 Squadron which is a combat Squadron - so, they'd most likely be picked up from Tindal first.. Thus, warning Williamtown who would be covering the East Coast of Australia (following the enemy movements by radar).
Now, if they came in from New Zealand airspace.. then NZ would give Williamtown a thumbs up and meet them at our borders - also, we have Navy ships cruising our coast so it wouldn't be such an easy fit to sneak right in. Also, given the enemy aircraft would need to travel over ocean and then return back to base, you can safely say they wouldn't go to the west coast of Australia unless they plan to land on an Aircraft carrier out at sea, which would be impossible given that our radar would pick that up.
We also have subs which are 'underwater floating radars'.
SOOOOO... to sum it up, you wouldn't be seeing a "Tomorrow when the war began" scenario in Australia that easy, if at all.