Four extinct volcanoes, which are thought to be about 50 million years old, have been discovered off the Sydney coast.

The volcano cluster, located about 5km beneath ocean surface, was discovered accidentally by a new Australian ocean-going research vessel hunting for lobsters and fish. The 94m Investigator found the volcanoes during its routine mapping exercise in search of baby lobsters.

The volcanoes are situated about 250km from Sydney and the largest of them is 1.5km across the rim. The cluster is about 20km long and 6km wide.

"There on the screen were these four incredible volcanoes looking like something off the front cover of a geology textbook...if you could drain the ocean it would be magnificent to see for a few seconds, it's a remarkable structure," said Iain Suthers, a professor at the University of New South Wales and a marine biologist.

"Now all of Sydney, all of Australia have it as part of their claim of sea floor and we never knew it was there," added the chief scientist.

They were not discovered before because Australia did not have the vessel to map the ocean floor at this depth.

"They tell us part of the story of how New Zealand and Australia separated around 40 to 80 million years ago. They'll now help scientists target future exploration of the sea floor to unlock the secrets of the Earth's crust," said volcano expert Richard Arculus, who works at The Australian National University, according to Sky News Australia.

Experts believe the volcanoes were created as part of a series of Earth's tectonic movements which also caused the separation of Australia and New Zealand. The voyage, which also found hotspots for larva fish and lobsters, was carrying 28 scientists from universities across Australia.

50-million-year-old volcanoes discovered off Sydney coast CSIRO