The purchase of the LHD's goes back over 20 years - the requirement was discussed in a White Paper in 2000 and confirmed in 206.
the aim was to project power across the Australasian area and to assist in humanitarian/police actions. Of course the possibility of doing other things (aka mission creep) was identified early on and there was discussion of using F-35B's.
The need for special coatings etc wasn't appreciated until they were well into development - around 2014/15. The assumption had always been that the F-35B would operate where a Harrier had operated. Even the USN were caught out and had to dock the USS "America" for nearly a year just after she commissioned to fix the problem.
Australia has only ordered F-35A's so any "carrier ops" would require B's or C's
Malcolm Davis in"The Strategist" in Jan 2019 advocated "taking the F-35 to sea" but admits that neither of the current vessels are suitable and it would cost at least $A 500 million EACH to modify. He suggested:-
" If the money is made available, a third LHD with a wing of between 12 and 16 F-35Bs, supported by a larger fleet of destroyers and frigates, is an option that should be on the agenda in any force structure debate."
The cost of such a program to produce a force of 16 aircraft seems make it highly unlikely that the F-35B will ever be acquired TBH - worse IMHO, is that people will start to think of the 3rd vessel as an Aircraft Carrier and put it in harms way in the same way that the RN confused Battle Cruisers with Battleships in 1914 which proved very costly at Jutland...................