"I had thought the Abrams were bought to be embarked (by ship or C-17) for infantry cover in the near island chains."
there are 17,00 islands in Indonesia - you'd better choose the right one to put a tank on as once the balloon goes up moving them about is likely to be a bit risky.
The Abrams is only on the list because the Army models itself on the US Army - so if they have tanks Australia needs tanks.
It's not necessarily so as a number of contributors here have pointed out