Australasia is mostly used to refer to countries like Australia, New Guinea, New Zealand, and other neighboring islands. In other words, Australasia is mostly used to describe a geographical region within Oceania i.e., a geographical region within many islands. The various countries or geographical areas that makeup Australasia are surrounded by a lot of islands.
For example, Australia is a continent, and it also exists as a country, however, it is part of the countries that are called Australasia because it is surrounded by the island of Tasmania towards its south. Also, New Zealand is surrounded by a lot of islands, and it has about 600 islands within it. At times, Australasia is used to refer to Australia, which is the smallest continent. Australasia has a geographical area which is bordered by the southern ocean to the south, and the Indian Ocean to the west. Its total mass area represents about 1.5 percent of the Earth's surface.