In the political sense, the term has little utility, as although Australia and New Zealand are both relatively wealthy, predominantly English-speaking countries and alike in many ways, they share little in common with the other nations in the area.
From a biological point of view, however, Australasia is a distinct region with a common evolutionary history and a great many unique plants and animals, some of them common to the entire area, others specific to particular parts but sharing a common ancestory.
The biological dividing line from Asia is the Wallace Line which represents the boundary between the two continental plates. This lies to the west of Borneo, Bali and Java.
In 1905, when Australia and New Zealand combined its best tennis players to compete in the Davis Cup international tourament, the team played under the name Australasia.