White Island has erupted sending plumes of ash and smoke high into the sky.
The eruption occurred at 2:15pm on Monday and eyewitnesses told Newshub it was big.
"It's huge," said operations manager of White Island Flights George Walker.
"A massive column of smoke with ash falling out of it," he told Newshub.
Walker said he understands people were on the volcanic island when it erupted.
"My understanding is there were a few people on the island when it happened, so hopefully everyone is safe."
St John has sent five helicopters to the island - 20 people are in need of medical assistance.
However, a local woman told Magic Talk 25 people are injured and some are missing.
"This is really serious," said one woman, named only as Leigh. Calling from Whakatane, she said emergency services were waiting for people to be rescued from the island.
"There are emergency services everywhere," she said.
She claimed 25 people were injured, 19 critically and some people unaccounted for. This information has not yet been confirmed by Newshub.
Leigh told Magic Talk it was a "sad day".
She also said a helicopter had been tipped over by the eruption and its occupants were forced on to a boat to escape.
"We've got people down here who have relatives out there so it's a pretty scary experience," she said.
Leigh said she had visited the island previously, and it was beautiful but she knew about the risk.
"There's always that danger - obviously there was no warning otherwise they wouldn't have been out there."
GNS volcanologist Brad Scott said the eruption was "significant".
"It's significantly affected the island and the main crater floor area," he said.
"There was an ash column to 12000 feet high - ash has blown to the east and ash has fallen into the sea," he said.
If there were people in the crater at the time, Scott says they would have had nowhere to go.
"There's not a lot of shelter. People who are on the island are quite exposed and we have significant concern for people that may have been on the island."
Prime Minister Jacinda Ardern confirmed around 100 people were on or around the island at the time of the eruption.
"Some of those are unaccounted for."
