A continent two-thirds the size of Australia has been found beneath the south-west Pacific Ocean, scientists reported in the journal of the Geological Society of America.

The land mass of 4.5 million square kilometres (1.74 million square miles) is 94 percent underwater and only its highest points - New Zealand and New Caldeonia - poke above the surface.

"It's rather frustrating for us geologists with the oceans being there," said Nick Mortimer, a geologist at GNS Science in Dunedin, New Zealand.

"If we could pull the plug on the oceans it would be clear to everyone we have mountain chains and a big high-standing continent above the ocean crust."

Mortimer was lead author of the paper titled "Zealandia: Earth's hidden continent" which says the new discoveries prove what had long been suspected.

"Since about the 1920s, from time to time in geology papers people used the word 'continental' to describe various parts of New Zealand and the Catham Islands and New Caledionia," Mortimer said.

"The difference now is that we feel we've gathered enough information to change 'continental' to the noun, 'continent'."

Mortimer said geologists early in the previous century had found granite from sub-antarctic islands near New Zealand and metaphormic rocks on New Caledonia that were indicative of continental geology.

If the recent discovery is accepted by the scientific community, cartographers will probably have to add an eighth continent to future maps and atlases.

"The paper we've written unashamedly sticks to empirical observations and descriptions," Mortimer said. "The litmus test will really be if 'Zealandia' appears in maps and atlases in five or 10 year's time.""Zealandia" is believed to have broken away from Australia about 80 million years ago and sank beneath the sea as part of the break up of the super-continent known as Gondwanaland.