Operational experience worldwide suggests that you need 2-3 carriers on the books in order to have one operational.
Australia currently spends, proportionately, 1/3 to 1/2 of that of NATO member states.
We also have the remote area bases in Queensland, NT and WA. The one in Queensland (Sherger) has been test activated a number of times, the other two at least once. these are essentially fully operational runways without electronic equipment. current estimates, with practical experience, are for 3-4 days before they become fully operational.
you can't operate jets from highways. firstly the road base and tarmac are not suitable for operating jets; secondly the amount of crap that gets sucked into the engines would lead to a breakdown very quickly. not to mention there are only so many roads up north- all would be needed for logistical efforts.
any interim fighter would require a big investment in spare parts, training and doctrinal adjustments. the decision to purchase the F/A -18F allows for use of a lot of existing parts for maintenance whilst reducing the amount of adjustment required for the pilots to operate the aircraft effectively.in my mind it would have been better to plump for some new build F-15's and F-16's ten years ago and then waited to see if the JSF F-35 was going to work. as it is we have put in nearly $500 million and have yet to have even an estimate of when they are coming.