I dont think you need an ASIC, as long as you have some type of Aviation Security Id card from Thailand, thats what all the other international airline pilots do, they dont have ASIC cards issued in OZ, they are issued from their country.
But sounds like an awesome holiday you are going to have, I flew an A36 from U-Tapo Direct to Darwin once had no spary and customs provided it for me, they are rather helpful too, long way tho.
Cheers