DANGER ZONE: The Rumble III underwater volcano has reduced in height by 100m after a 'catastrophic' eruption, according to scientists.

There has been an enormous eruption in an underwater volcano about 300 kilometres from the Bay of Plenty and similar in size to Mt Ruapehu.

Rumble III, which is part of the 250-kilometre South Kermadec Ridge, rises 2300 metres from the sea floor. Until 2007, its summit was within about 200m of the sea surface.

However, scientists on a research ship that has just spent two weeks investigating underwater volcanoes have found a "startling change" in the shape of the summit, which has reduced in height by about 100m.

Cornel de Ronde, co-chief GNS scientist on the voyage, said the volcano had endured a "fairly catastrophic" eruption.

A map made in 2007 showed an 800m-wide crater near the top, but the new map now showed the crater had been filled and the nearby summit cone had shrunk.

"This suggests there has been a major eruption that collapsed the summit cone and filled the adjacent crater," Dr de Ronde said.