If you are not careful enough to look at the two words (Australasia and Australia) very well, you might easily think they are the same. And aside from the seeming difference in their spellings, both words do not have the same meaning. Australia is the smallest of all the continents that are in the world. It holds the second to the last position in the list of continents with a high population.
Australasia, on the other hand, is a term that can be used to refer to countries like New Zealand, Australia, New Guinea, and other islands. The major reason why Australia and Australasia are not the same is that Australia is a continent due to the fact it sits on its own tectonic plate, and it is also a country, whereas Australasia is just a term describes countries that within Oceania. If you use Australasia in place of Australia, you will be missing a lot of things.