A cluster of extinct volcanos that could shed new light on how New Zealand was formed have been discovered about 240 kilometres off the coast of Sydney.
Australia's national science agency, CSIRO, was searching for nursery grounds for larval lobsters when research vessel Investigator found the volcanoes nearly 5km under the ocean surface.
The volcanoes are believed to be about 50 million years old.
Scientists believe they will tell part of the the story of how New Zealand and Australia separated tens of million years ago.
Iain Suthers from the University of New South Wales is the chief scientist on the CSIRO voyage. He said it was an exciting, serendipitous discovery.
"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 just a few seconds - it's just a remarkable structure.
"Now all of Sydney, all of Australia, have it as part of their claim of the sea floor - and we never knew it was there."
Professor Richard Arculus of the Australian National University is a world-leading expert on volcanoes. He said the discovery told part of the story of how New Zealand and Australia separated 40 - 80 million years ago.
Mr Arculus said the volcanoes would help scientists target future exploration of the sea floor.
The volcanoes are calderas, which form after a volcano erupts and the land around it collapses, forming a crater.
The four newly discovered calderas form a cluster 20km long and 6km wide, with the largest 1.5km across and 700m high.
Investigator is the first Australian research vessel capable of scanning the deep part of the ocean where they lie.