GNS Science says a sea of pumice floating towards New Zealand comes from a previously inert caldera called the Havre Volcano near Curtis Island in the Kermadecs.
The pumice is moving southwards.
Principal scientist Cornel de Ronde says undersea volcanoes erupt from time to time, but activity from this one has not been recorded before.
He says there are far more volcanoes under the ocean than on land.
"When these particular types of volcanoes erupt," Mr de Ronde says, "they tend to form a lot of pumice and the pumice is very light so it floats to the surface, so there's been a number of these pumice rafts over the last several years."
It was detected with the help of French seismologists in Tahiti.