Prime Minister Jacinda Ardern said about 100 tourists were on or near White Island when it erupted in the afternoon.
“Some of those, at this stage, are unaccounted for,” she said. ‘”A number of people are reportedly injured and are being transported to shore.”
She said the incident appeared to be “very significant.”
“All our thoughts are with those affected,” she said.
Ardern said there were no confirmed fatalities.
St John medical responders said in a statement they believed there were 20 people on the island who were injured and in need of medical treatment. It said it had dispatched seven helicopters to the island with paramedics aboard.
The GeoNet agency said a moderate volcanic eruption had occurred and raised its alert level to four, on a scale where five represents a major eruption.
White Island sits about 50 kilometers (30 miles) offshore from mainland New Zealand. There will be questions asked as to why tourists were still able to visit the island after scientists recently noted an uptick in volcanic activity.
White Island is northeast of the town of Tauranga on North Island, one of New Zealand’s two main islands. Police were asking people to avoid areas on the North Island that were close to the eruption, including the Whakatane Heads and Muriwai Drive areas.
GeoNet says it is New Zealand’s most active cone volcano and about 70 percent of the volcano is under the sea.
Twelve people were killed on the island in 1914 when it was being mined for sulphur. Part of a crater wall collapsed and a landslide destroyed the miners’ village and the mine itself.
The remains of buildings from another mining enterprise in the 1920s are now a tourist attraction, according to GeoNet.
The island became a private scenic reserve in 1953, and daily tours allow more than 10,000 people to visit the volcano every year.
The island is also known by the indigenous Maori name Whakaari.