Deep sea volcanoes found off Sydney

Four volcanoes discovered off the coast of Sydney are not at risk of erupting but could hold answers about Australia's rich mineral deposits.

The cluster of volcanoes, thought to be about 50 million years old, were found by chance about 250km east of Sydney during a voyage by the CSIRO's new research vessel, Investigator.

It's exciting news, ANU geologist Richard Arculus said.

"This is the first time these volcanoes have been seen," he told AAP on Monday.

"It proves yet again that we know more about the topography of Mars than we do the sea bed in our own backyard."

Sonar images of the in-line volcanoes show large craters in the top of each, which Professor Arculus said proves they have all erupted, possibly in an explosive manner.

"But there's no real chance (they) are going to erupt again ... these guys have been dead for a long time.

"And volcanic activity in the Tasman Sea is extinct."

The Investigator was using its multi-beam sonar arm to map the seabed while on a voyage looking at the breeding ground of lobster, when it found the volcanoes.

It is the first time such technology has been used in the area, Prof Arculus said, adding that the discovery will spark calls for the volcanoes to be further analysed.

"From a purely scientific point of view, it's learning more about the past of Australia and New Zealand and the nature of the stuff below the crust of the earth," he said.

"It could provide information about why the east of Australia stands high, and about the nature of the country's mineral deposits, which always drives curiosity."

The volcanoes and their activity could also provide information about a deep sea plateau, Lord Howe Rise, which split from Australia up to 100 million years ago and now stretches between New Caledonia and west of NZ.