It is not the defence of the mainland that creates problems. For example that unprotected Western Australian coastline that you pointed to is substantially covered if you take into the account the existence of bare bases such as Curtain.

No, range is more of an issue because Australia's current defensive posture is to try to stop any potential enemy in what is called the air-sea gap. That means interdiction at long distances off the coastline. With aircraft carriers being financially out of reach that leaves long range aircraft as the only viable option. The f-35 is going to be a disaster for Australia's defence, we can only hope that the US honours the goodwill that the purchase of those systems was supposed to procure. Of course, they're going to be flying the same crappy airframes, so meh.

Just one last thing in defence of the JSF though. It's unique capabilities in takeoff / landing make cheaper aircraft carriers a real possibility. Of course Australia hasn't actually tried to procure any such thing so it's only a theoretical, but if tensions were to start to mount in Asia that is something that could be rectified relatively quickly...