Scientists have confirmed that White Island has erupted and a new vent has formed on the volcano.
The island, off Whakatane in the Bay of Plenty, has shown increased activity this week, but on Tuesday tremors dropped sharply about 11.30pm.
It had been unclear whether there was an eruption on the island, but on Thursday afternoon scientists from GNS Science confirmed this had taken place after they were able to visit the island.
"This is the first ash emission from White Island since February 2001 and represents the start of a new phase of volcanic activity at White Island," volcanologist Brad Scott said.
A new vent on the southwest side of the mountain has been formed by the eruption.
Mr Scott says the eruption is considered weak, sending ash just 200 to 300 metres above the new vent. The black ash is falling on the wall of the main crater to the west of the vent.
The risk to visitors to White Island is now at its highest level since 2001.
"Additional hazards to visitors to the island now include the health effects of volcanic ash and acid gas exposure, including respiratory issues, skin and eye sensitivity to acid gases," Mr Scott said.
"Explosive eruptions can occur at any time with little or no warning."
The eruption happened less than 24 hours after an eruption at Mt Tongariro in the central North Island.
The mountain erupted about 11.50pm on Monday, spewing ash and prompting a threat warning for the central North Island.
Some flights were cancelled and nearby roads and tramping tracks were closed due to ash fall.
Rocks fell within 1km of the eruption, damaging Ketetahi Hut - one of four trampers' huts on the mountain - but there have been no reports of injuries.