A huge undersea volcano not far from the coast has dramatically reduced in size after partly collapsing more than 100 metres toward the ocean floor.

National Institute of Water and Atmospheric Research scientists found that a volcanic cone on Rumble III, 200km northeast of Auckland, had crumbled. The volcano's highest point had dropped 90m further below the Pacific Ocean, and in some places the volcano had slipped as much as 120m.

Marine geologist Richard Wysoczanski said the collapse was caused by an eruption some time in the last two years.

"It's a huge amount of debris - if you can imagine that falling off the top of Mt Ruapehu, which is a similar size [to Rumble III], then you get an idea of the drop."

The 2300m Rumble III forms part of the Kermadec Ridge, a jagged chain of 30 large underwater volcanoes that ascend from the ocean floor between New Zealand and Tonga.

The volcano was last mapped by Niwa in 2002 using multibeam technology, a sonar-like system that gives scientists a digital image of its terrain.

In June this year the collapse on the western flank of the volcano was confirmed by Niwa on an oceanographic voyage on the RV Tangaroa.

Niwa principal scientist Geoffrey Lamarche said that the discovery provided a rare insight into the movements of the seabed over a short timespan.

"We know the movements of volcanoes over millions of years. But we often struggle to understand them in the human scale. We can now say that in the space of eight years we can have significant pieces of seafloor moving hundreds of metres in height."

He said they could use the information to gauge the potential impact of a larger collapse on New Zealand's shores. Rumble III's peak was shallow - now 300 metres under the surface - and another, greater collapse on the western side could send a tsunami towards Auckland and the Bay of Plenty.

Dr Wysoczanski said a number of eruptions and landslips in the last 10 years showed New Zealand's seascape was more changeable than first thought.

He pointed to eruptions at Monowai in 2008, Raoul Volcano in 2006, and White Island in 2000 and 2001.

"There's no doubt some of these slips can cause tsunami. This one was quite small, but it sits right on top of a much older, larger structure which would've probably sent a tsunami towards New Zealand. It's pointing [southwest] - right towards us.

"At the moment we just don't understand how often these collapses occur and what size tsunami they could generate."

The Tangaroa trip focused on geophysical surveying of the Kermadec Arc seafloor and the seafloor massive sulphide deposits that sometimes developed over hydrothermal vents. The volcanoes and vents were rich in iron, lead, zinc, and copper, with lesser concentrations of gold and silver.

Dr Wysoczanski said the surveys informed the Government's policy strategy in an area with both a high mineral and biodiversity value.