At 4.9 million square kilometres of land mass, Zealandia would be the world's eighth and smallest continent. The scientists presented their findings in the study Zealandia: Earth's Hidden Continent, which was published in GSA Today, the journal of The Geological Society Of America. They say Zealandia and Australia come remarkably close to each other across the Cato Trough, off the coast of Queensland. At that point the continental crusts of the two continents are just 25 kilometres apart. According to the study, the 94 per cent of Zealandia currently submerged broke away from Australia and sank 60 to 85 million years ago.

The scientists contend the data shows a substantial part of the south-west Pacific consists of a continuous expanse of continental crust. They say it is large and separate enough to be considered a continent in its own right. The name Zealandia was coined by geophysicist Bruce Luyendyk in 1995 as a collective name for New Zealand, the Chatham Rise, Campbell Plateau and Lord Howe Rise. At the time it was believed to possess three of the four necessary qualities required for continent status. These include the presence of different rock types and crucially "the high elevation relative to regions floored by oceanic crust".

A recent discovery using satellite technology and gravity maps of the sea floor have revealed that Zealandia is a large unified area, fulfilling all four requirements. "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 of the study say. Nick Mortimer, of GNS Science, told Fairfax Media: "The significance of the recent paper is that it is the first peer reviewed published paper properly defining and proposing Zealandia. As such we hope it will be the 'go to' reference on the subject." Maria Seton from the University of Sydney, one of the authors of the new study, said the concept of Zealandia as a continent had been around since the 1990s in a generalised sense. "We think more of Zealandia was once above water but we don't know how much," Dr Seton said.

"For instance sites we have dredged that are two kilometres deep were once close to sea level. We have also seen old river systems of Zealandia on the sea bed. "Zealandia contains geological information about eastern Gondwana, which included Australia." Zealandia has a continental crust thickness ranging from 10 to 30 kilometres, getting up to more than 40 kilometres under parts of New Zealand's South Island. While most of it is thinner than the 30 to 46 kilometres typical of most continents, it is everywhere thicker than the crust of the ocean basins, which is about seven kilometres. "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. Zealandia is not unique among continents in being mostly submerged. "An ice-free, isostatically corrected West Antarctica would also largely be submerged," the authors say. Zealandia once made up about 5 per cent of the area of Gondwana, from which it started to separate as a ribbon continent about 4000 kilometres long, the paper says. It has since gone through substantial deformation to end up in its present shape and position.

"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 continent," the authors say. "Currently used conventions and definitions of continental crust, continents, and microcontinents require no modification to accommodate Zealandia." The political and economic implications of a new continent would be manifold, with the question of clearly defining what belongs to New Zealand and Australia particularly salient in light of offshore mining in the area. A six-year study by the GNS Science research institute in New Zealand has revealed that tens of billions of dollars worth of fossil fuels could be located off-shore in the region. Loading

As well as GNS staff who contributed to the paper, other authors come from the University of Sydney, Victoria University and the Service Geologique de Nouvelle Caledonie. An elevation map of Zealandia and nearby Australia. Credit:GSA Today with stuff.co.nz; Telegraph, London