Tourists are being warned of the risk on White Island as volcanic activity reaches its most vigorous in decades.
The island, off the Bay of Plenty coast, had a small eruption late last year and after a lull afterwards, there is now more unrest.
GNS Science vulcanologist Brad Scott said small scale hydrothermic eruptions were occurring through the island's lake every few seconds.
"The last time I saw stuff as strong as this was probably in the 1999-2000 period."
White Island had a significant eruption in mid-2000 following that activity increase but Mr Scott said there was no guarantee it would happen again.
"This could just curl up and go away, or it could lead to an increase in activity."
White Island is uninhabited but some tourists do visit the island; Mr Scott estimated there were about 100 on the island yesterday.
Today, Bay of Plenty Civil Defence Emergency Management Group said there was some risk to people visiting the island and travelling nearby.
They were advised that an eruption could occur at any time with little or no warning.
Civil Defence was keeping local tour operators and other parties informed of the situation.