GNS Science says a sea of pumice floating towards New Zealand comes from the Havre volcano near Curtis Island in the Kermadecs.
The pumice, forming a raft of 25,000 square kilometres, was first seen about 85 nautical miles west-south-west of Raoul Island last week.
Scientist Cornel de Ronde says its origin was detected with the help of French seismologists in Tahiti.
He says the volcano hasn't erupted before, but at a depth of 1100 metres it's not a danger to the public.
Mr de Ronde says it's similar to White Island, which erupted last week.