GNS Science has issued a volcanic alert for White Island in the eastern Bay of Plenty.
It says the island's crater has started to refill and gases are vigorously steaming through it.
GNS says tremors are not uncommon at White Island but the latest activity is the first change in the past few years.
It says the warning has also been prompted by a rise of about five metres in the crater's lake last week.
GNS spokesperson Brad Scott says it's a low-level alert but eruptions can occur at any time with little or no warning and the public should take extra caution when approaching the crater.