Australasia, a region of Oceania, comprises Australia, New Zealand, neighbouring islands in the Pacific Ocean and, sometimes, the island of New Guinea (which is usually considered to be part of Melanesia). Charles de Brosses coined the term (as French "Australasie") in "Histoire des navigations aux terres australes" (1756). He derived it from the Latin for "south of Asia" and differentiated the area from Polynesia (to the east) and the southeast Pacific (Magellanica). The bulk of Australasia sits on the Indo-Australian Plate, together with India. 

Physiographically, Australasia includes New Zealand, Australia (including Tasmania), and Melanesia: New Guinea and neighbouring islands north and east of Australia in the Pacific Ocean. The designation is sometimes applied to all the lands and islands of the Pacific Ocean lying between the equator and latitude 47° south. Physiographically, Australasia includes the Australian landmass (including Tasmania), New Zealand, and New Guinea. The independent country of Papua New Guinea also includes approximately 600 offshore islands. 

Most of Australasia lies on the southern portion of the Indo-Australian Plate, flanked by the Indian Ocean to the west and the Southern Ocean to the south. Peripheral territories lie on the Eurasian Plate to the northwest, the Philippine Plate to the north, and in the Pacific Ocean – including numerous marginal seas – atop the Pacific Plate to the north and east.
Why?
 sometimes, the island of New Guinea (which is usually considered to be part of Melanesia).