Scientists say the collapse of a massive undersea volcano off the coast of the North Island shows New Zealand could be at a high risk of tsunami.
Marine geologists from the National Institute of Water and Atmospheric Research (NIWA) have confirmed a major eruption about 200km northwest of Auckland occured some time in the last two years.
The Rumble 3 volcano, which is larger than Mount Ruapehu, has lost about 120 metres in height.
NIWA principal scientist Dr Geoffroy Lamarche says the undersea mapping shows the seabed is more active than was previously known.
"We don't want to be scaremongering here, but definitely any large displacement of (the) seafloor will pull the water down.
"At the surface it will also generate a wave, and this collapse, we know because we've mapped it, this collapse is looking south-west, which is directly toward the New Zealand coast."
Dr Lamarche says scientists will use the data to model the potential impact of a tsunami on Auckland and Bay of Plenty.