An undersea volcano, aptly named Rumble lll, has partly collapsed, the National Institute of Water and Atmospheric Research (Niwa) says.

The west side of the volcano, on the Kermadec Ridge, 200km northeast of Auckland, collapsed over the past several years, reducing its height by 120 metres, Niwa marine geologist Richard Wysoczanski said.

The collapse was confirmed during an oceanographic voyage during May and June this year.

"Our seabed is a lot more active than we thought," Dr Wysoczanski said.

"These volcanoes, as well as the non-volcanic ridges, can also cause landslides that could potentially generate a tsunami that would impact on New Zealand."

Dr Wysoczanski said the first leg of the voyage focused on geophysical surveying of the Kermadec Arc seafloor and seafloor massive sulphide deposits that sometimes develop over hydrothermal vents.

Rumble lll is one of more than 30 large submarine volcanoes on the Kermadec Arc which are rich in iron, lead, zinc and copper.

"We are investigating these volcanoes and mineral deposits to determine their size and the biological communities they support," Dr Wysoczanski said.

"The research will provide industry and government agencies with the information they need to make the best decisions based on the biological and physical characterisation of the sea floor."

Glassy, black basalt rock was also collected from the sea floor.

Dr Wysoczanski said it was some of the youngest known rock from the Kermadec region.

"(It) essentially tell the origins of the volcanoes. Scientists can tell from the type of rock produced whether they have come from a deep magma chamber or from a shallow magma chamber, and how explosive the eruption may have been," he said.

"We did 23 dredges on the Kermadec Arc, including the Kermadec Ridge, which has never been dredged before. We knew it was volcanic but we didn't know what sort of volcanism - so it was quite exciting."