Should re-use the Ceafar and mk41 from the retiring ANZACsExactly this. In the fog of war, with competing priorities and given the limited number of escorts operated by the RAN (and that's before attrition), the chances of one of these major platforms being caught alone seems somewhat likely.
One of the LHDs, or Choules for that matter, being taken out with crew plus on an onboard army contingent (say several hundred) would be devastating for the nation and potentially even influence the country's will to fight. Its staggering that these assets have been left unprotectef by successive governments.
At the minimum, CEAFAR, 1xMk41 with ESSM plus point defence cannons.