Researchers have stumbled across four extinct volcanoes likely to be more 50 million years old around 250 kilometres off the coast of Sydney.

The scientific team -- led by marine biologist, Professor Iain Suthers from the University of New South Wales -- were routinely mapping the seafloor searching for the nursery grounds of larval lobsters when the volcanoes were found in 4,900 metres of water.

Four volcanoes in the cluster are known as "calderas", which form after a volcano erupts and land around them collapses, forming a crater, UNSW said. The cluster is 20km long and 6km wide with the highest point rising up to 3,998 metres.

Professor Richard Arculus, a world leading expert on volcanoes, said these volcanoes are like 'windows into the seafloor.'

"They tell us part of the story of how New Zealand and Australia separated around 40 to 80 million years ago and they'll now help scientists target future exploration of the sea floor to unlock the secrets of the Earth's crust," he said.

He said they haven't been found before now because sonar on the previous Marine National Facility (MNF) research vessel, Southern Surveyor, could only map the sea floor to 3,000 metres. This left half of Australia's ocean territory out of reach, he said.

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Australia's vast ocean territory is now within reach with the new MNF vessel, dubbed 'Investigator.'

"The voyage was enormously successful. Not only did we discover a cluster of volcanoes on Sydney's doorstep, we were amazed to find that an eddy off Sydney was a hotspot for lobster larvae at a time of the year when we were not expecting them," Professor Suthers said.

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A larval lobster specimen collected during the expedition

The research voyage - which departed Brisbane on June 3 and concluded in Sydney on June 18 - involved 28 scientists from UNSW, La Trobe University, the University of British Columbia, the University of Sydney, the University of Auckland, the University of Technology Sydney, and Southern Cross University.

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