GNS Science says White Island has been emitting ash and the alert level has been raised.
The Volcanic Alert level was raised to 2 after craters vented ash on Saturday. The alert scale runs from 0 to 5.
The Bay of Plenty island has been deemed at risk of erupting since ash emissions and small eruptions began last August.
GNS Science said volcanologists will be monitoring the volcano closely.
Volcanologist Arthur Jolly said a flight will be made over the craters in the next few days.