It turns out New Zealand isn't a couple of small islands at the bottom of the world. It's actually a continent - most of which just happens to be under the sea.

The case for Zealandia is made in a paper with considerable input from GNS Science, published in GSA Today, the journal of The Geological Society Of America.

According to the authors, Zealandia and Australia come remarkably close to each other across the Cato Trough, off the coast of Queensland.

GEOLOGICAL SOCIETY OF AMERICA Zealandia is large and separate enough to be considered an actual continent.

At that point, the continental crusts of the two continents are just 25km apart.

READ MORE: Searching for the lost continent of Zealandia

GEOLOGICAL SOCIETY OF AMERICA Ninety-four percent of Zealandia is under the sea.

In total, Zealandia covers about 4.9 million sqkm, of which 94 per cent is underwater.

It would all be revealed "if you pulled the plug on the world's oceans", the paper's principal author and GNS geologist Nick Mortimer says.

The Chatham Islands and subantarctic islands are part of Zealandia, but Fiji and Tonga aren't.

It's also shared with Australia, as well as France, given that New Caledonia is another part of Zealandia.

But there are no territorial implications, with maritime boundaries long agreed upon.

"It's mainly a better and more accurate description of what's there in this corner of the planet," Mortimer says.

Water depth in the ocean around Zealandia is about 4000 metres deep, while the shelves around the Chathams, subantarctic islands and New Caledonia are about 1000 metres.

"There's a big step off the edge around Zealandia," Mortimer says.

"Most continents have a big landmass and narrow continental shelf, whereas Zealandia has a small landmass and a very wide continental shelf."

The highest point on the continent is Aoraki-Mt Cook.

The authors contend the data shows a substantial part of the southwest Pacific consists of a continuous expanse of continental crust.

It is large and separate enough to be considered an actual continent.

"This is not a sudden discovery but a gradual realisation; as recently as 10 years ago, we would not have had the accumulated data or confidence in interpretation to write this paper," the authors say.

Zealandia has a continental crust thickness ranging from 10-30km, getting up to more than 40km under parts of the South Island.

While most of it is thinner than the 30-46km typical of most continents, it is everywhere thicker than the crust of the ocean basins, which is around 7km.

"Being more than 1 million sq km in area, and bounded by well-defined geologic and geographic limits, Zealandia is, by our definition, large enough to be termed a continent," the paper says.

It is 12 times bigger than Mauritia and six times bigger than Madagascar, and about the same area as greater India.

It is not unique among continents in being mostly submerged: "An ice-free, isostatically corrected West Antarctica would also largely be submerged."

Zealandia once made up about five per cent of the area of Gondwana, from which it started to separate as a ribbon continent about 4000km long, the paper says. It has since gone through substantial deformation to end up in its present shape and position.

"It's been pretty panelbeaten on its journey," Mortimer says. "It's had 100 million years of stretching and moving. Now there's a huge plate boundary through it."

The continent of Zealandia provided a useful context for looking at a few things. One was New Zealand's offshore resources and another was the country's native flora and fauna.

"Based on various lines of geological and geophysical evidence, particularly those accumulated in the last two decades, we argue that Zealandia is not a collection of partly submerged continental fragments but is a coherent 4.9 Mkm2 (square kilometre) continent," the paper says.

"Currently used conventions and definitions of continental crust, continents, and microcontinents require no modification to accommodate Zealandia."

Mortimer says there is no body that can declare Zealandia a continent but it is increasingly being used in scientific literature by geologists and biologists.

"We've taken the time here to write a formal scientific paper on it."

In 2014, Mortimer and GNS geologist and paleontologist Hamish Campbell wrote a book, now out of print, about Zealandia.

The name Zealandia was first proposed by geophysicist Bruce Luyendyk in 1995 as a collective name for New Zealand, the Chatham Rise, Campbell Plateau, and Lord Howe Rise.

As well as GNS staff who contributed to the paper, other authors come from Victoria University, the University of Sydney and the Service Geologique de Nouvelle Caledonie.