thumb|upright=1.35|Map showing the Indo-Australian plate (IA) and other major plates
thumb|upright=1.8|The Indo-Australian plate, shown as its two subdivisions: the [[Indian plate (red) and the Australian plate (orange)]]
The Indo-Australian plate is or was a major tectonic plate. It is in the process of separation into two or three plates, and may be currently separated into more than one plate. It contains the continent of Australia, its surrounding ocean and extends north-west to include the Indian subcontinent and the adjacent waters.
Formation
It was formed by the fusion of the then Indian and the then Australian plates approximately 43 million years ago. The fusion happened when the mid-ocean ridge in the Indian Ocean, which separated the two plates, ceased spreading.
Regions
Australia-New Guinea (Mainland Australia, New Guinea, and Tasmania), the Indian subcontinent, and Zealandia (New Caledonia, New Zealand, and Norfolk Island) are all fragments of the ancient supercontinent of Gondwana. As the ocean floor broke apart, these land masses fragmented from one another, and for a time these centers were thought to be dormant and fused into a single plate. However, research in the early 21st century indicates plate separation of the Indo-Australian plate may have already occurred.
A third plate, known as the Capricorn plate, may also be separating off the western side of the Indian plate as part of the continued breakup of the Indo-Australian plate.
Separation
There is good evidence that the Indo-Australian plate is in the process of separation into new plates. and that the Indian plate and Australian plate may have been separate since at least .
Contemporary models suggest at present there is a deformation zone between the Indian and Australian plates, with both earthquake and global satellite navigation system data indicating that India and Australia are not moving on the same vectors northward. Studies show the Ninety East Ridge has active faulting along its whole length so that while the simplest explanation is that the Indian and Australian plates have already separated here, it remains possible that only the Capricorn plate has separated from them.