A wonderland of extinct volcano craters, ancient river beds and rocky reefs teeming with life has been found just off the NSW coast as part of a mapping process that could reveal other treasures.

The first section of the sea floor off Wollongong - from Bellambi Point to the Five Islands - to be subjected to the government's digital mapping program found reefs surprisingly covered 60 per cent of the region.

Of particular interest was the discovery of volcanic features, known as diatremes, that form as magma rises through cracks in the earth's crust and meets ground water.

The super-heated water vapor and volcanic gases expand rapidly, causing explosions that leave behind shallow craters, scientists with the Office of Environment and Heritage said.

"We didn’t know the undersea craters were there," David Hanslow, the senior team leader of OEH's Coasts and Marine Science, said.