Scientists have located an undersea volcano that erupted in July, creating a pumice raft the size of Canterbury.
The eruption of the Havre volcano in the Kermadecs, was strong enough to breach the ocean surface from a depth of more than 1km.
The Kermadec chain stretches from Tauranga to Tonga.
NIWA expedition leader Dr Richard Wysoczanski said the seafloor has been mapped from a ship with an echo sounder to obtain an idea of the physical effect of the eruption.
He said several cubic kilometres of material has settled on the sea floor, raising it by 10 metres.