Something which people frequently seem to forget, at least amongst those not actually involved in analyzing the Oz defences, is the importance of some of the broad area surveillance systems. Take JORN and SECAR for instance, both provide a ground-based detection system which covers a wide area. While neither system can provide 'target quality' data, with the use of the appropriate C4ISR systems, a contact from JORN and/or SECAR (more likely JORN since that has longer range) can then que other assets to either investigate and/or intercept.
In terms of providing air defence, it is not just about who has the "best" or most aircraft, but really who can make the best use of them. To make the best use of something like combat aircraft, whoever has the most/best information about the situation will typically have the advantage.
Now, with others having posted about the potential "threat" of India and/or China, that might be something Australia should be concerned about, once it actually exists. At present, neither nation could carry out any real practical airstrike against Australia. AFAIK the military aircraft which both countries that is capable of actually reaching Australia (and perhaps even returning) is confided to long-ranged transports, not bombers, strike or attack aircraft. Now, if both/either country began making hostile noises directed at Australia, and if they suddenly began building up a long-ranged strike capability or suddenly acquired air bases within easy reach of the Australian mainland, then perhaps things would be different.
At present though, either larger, multi-engined bombers would be required, or a dramatic expansion of the AAR fleets of the respective nations. That or an ASEAN nation like East Timor, Indonesia, Papua New Guinea or perhaps Malaysia would need to either be overrun or lease an airbase and appropriate facilities to either China or India. [
In short, there is a great deal more involved in defending Australian airspace than just nn numbers of yy fighters.
-Cheers