Researchers believe Zealandia was not always as deep under the sea as it is now.

The mostly sunken continent of Zealandia, which surrounds New Zealand, used to be closer to the surface of the sea and had a very different climate in the past, researchers believe.

A team of 32 scientists from 12 countries has just finished a nine-week voyage to study the undersea continent, drilling deep into the seabed at six sites in water depths of more than 1250 metres.

More than 8000 fossil specimens were studied, with several hundred species identified.

IODP International research drilling ship Joides Resolution has just finished a nine-week expedition to study Zealandia.

"The discovery of microscopic shells of organisms that lived in warm shallow seas, and of spores and pollen from land plants, reveal that the geography and climate of Zealandia were dramatically different in the past," expedition co-chief scientist Gerald Dickens, of Rice University in the US, said.

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The new discoveries showed the formation 40 to 50 million years ago of the Pacific ring of fire, an active seafloor zone along the perimeter of the Pacific Ocean, caused dramatic changes in ocean depth and volcanic activity and buckled the seabed of Zealandia.

The fossil discoveries proved Zealandia - now more than 1km under the sea - was not always as deep under the waves as it was now, Dickens said.

Victoria University of Wellington professor Rupert Sutherland, and also co-chief scientist of the expedition, said researchers had believed Zealandia was submerged when it separated from Australia and Antarctica about 80 million years ago.

"That is still probably accurate, but it is now clear that dramatic later events shaped the continent we explored on this voyage," Sutherland said.

"Big geographic changes across northern Zealandia, which is about the same size as India, have implications for understanding questions such as how plants and animals dispersed and evolved in the South Pacific.

"The discovery of past land and shallow seas now provides an explanation. There were pathways for animals and plants to move along."

The expedition to drill Zealandia was carried out on board research ship Joides Resolution by researchers affiliated with the International Ocean Discovery Programme (IODP).

According to the US National Science Foundation (NSF), a key IODP sponsor, Zealandia was confirmed as a continent earlier this year.

NSF division of ocean sciences programme director Jamie Allan said the expedition offered insights into Earth's history, ranging from mountain-building in New Zealand to shifting movements of Earth's tectonic plates, to changes in ocean circulation and global climate.

The expedition would hopefully shed light on several profound Earth history problems of global significance, and the history of Zealandia over the past 60 million years, Allan said.

One key problem was to do with changing ocean circulation patterns and another was how subduction - where one tectonic plate goes under another - started.

Between 40-50 million years ago there was a profound shift in the orientation of the Earth's tectonic plates, with the Pacific Plate changing direction significantly. It began to dive down beneath Asia and underneath the eastern edge of Zealandia.

That was essentially the origin of the Pacific rim of fire, and because Zealandia was made up of continental rocks which were less dense than oceanic crust it kind of "floats", Allan said.

The expedition researchers were particularly interested in the Kermadec-Tonga Trench - which stretches northeast of New Zealand up past Tonga, and is one of the deepest places on Earth. It is where the Pacific Plate dives, or subducts, underneath the crust.

The subduction of the Pacific Plate caused the uplift of Zealandia, and a consequence of that was a change to ocean circulation patterns, Allan said.

That helped lead to the isolation of Antarctica by the Antarctic Circumpolar Current, which helped prevent heat being transferred from the rest of the Earth to Antarctica.

Crocodiles had lived in Antarctica 35 million years ago, now there were thick sheets of ice, Allan said.

Studies of 2500m of sediment cores obtained during the expedition will focus on understanding how Earth's tectonic plates move and how the global climate system works. For example, fossils in the samples can show how warm and deep the sea was at various times. That work will take some years.

Records of Zealandia's history are expected to provide a sensitive test for computer models used to predict future changes in climate.

GNS Science said Zealandia was a fragment of the ancient continent of Gondwanaland. About half the size of Australia, it was initially all land but slowly sank as the Tasman Sea, which began to open about 83 million years ago, grew.

It continued to be stretched and thinned until about 23 million years ago, when it was mostly under the sea.