Wiki says:
Australasia, a region of Oceania, comprises Australia, New Zealand, the island of New Guinea, and neighbouring islands in the Pacific Ocean. Charles de Brosses coined the term (as French Australasie) in Histoire des navigations aux terres australes (1756).
Austrasia, as I understand it, is one of those words coined by Fr Chazal, to include AU, NZ and other places, including possibly Singapore, served by the priests. But don't quote me because I am not up to scratch on the Resistance, living in a too far flung region to attend.
Back to the subject in hand!