Zealandia is a long, relatively narrow fragment of continental crust that split from Antarctica early in the Cretaceous Period (about 120 million years ago) and from Australia toward the end of the Cretaceous (about 80 million years ago as the Tasman Sea opened).  This continent, 93% of which is submerged beneath the sea, has since drifted northeastward and now stretches NW to SE, from the tropics, north of New Caledonia, to the sub-Arctic zone, southeast of New Zealand; while most of Zealandia is hidden by ocean waters, it covers an area half the size of Australia.

Northern Zealandia, which is composed of two parallel ridges separated by a long, narrow graben, lies on the Australian Plate while Southern Zealandia (from New Zealand's southern island southward) lies on the Pacific Plate; compression between these plates forced up the Alps on New Zealand's southern island and subduction of the Pacific Plate beneath the Australian Plate produced the volcanoes on New Zealand's northern island.  New Caledonia, New Zealand and numerous small islands are the only segments of Zealandia that currently poke above the southwestern Pacific.

Since rifting from Australia and Antarctica, the portion of Zealandia visible above the ocean has expanded and contracted as sea levels have fallen and risen, respectively.  As one might expect, marine sediments are thus found on New Caledonia and New Zealand while fossils of Mesozoic plants and animals from Australia and Antarctica have been discovered on those land masses, attesting to the origin of Zealandia.