Austrailians came from England. They were English convicts brought to Austrailia for imprisonment or something.
Australia, in addition to having been an English colony, is home to the more native Aboriginals. It is these indigenous peoples which comprise the context for the word "Australasian"--which is primarily a geographical word referring to Australia, New Zealand, and other islands in the area--as a racial term describing a broad subset of the human species. The word
Australia is based on the Latin word for "southern." For instance, the
aurora australis describe the southern lights, and
Australia itself was described a southern landmass--not very creative, but certainly applicable.
Given how badly we "Caucasians" have trod on native peoples in our adventures around the globe, it's just a bit unsavory to completely forget that Australia had people before the English arrived.