Police said 23 people – some of whom were injured – had been rescued from White Island, a popular tourist destination, on Monday.
But it is unclear how many people remain unaccounted for, said police deputy commissioner John Tims, adding that it was currently too dangerous for police and rescue services to travel to the island.
There has been no contact with those missing on the island, Tims said.
However, it is believed that less than 50 people were on White Island when the volcano erupted.
St. John Ambulance said that up to 20 people were believed to have been injured in the eruption, adding that a mobile triage unit was on its way. The organisation said it had sent seven helicopters to the island with medics onboard.
New Zealanders and foreign tourists are believed to be involved in the incident, which saw the island erupt with a large plume of ash and steam.
Aerial images showed groups of people walking towards and away from the rim inside the crater, from which white vapour constantly billows, in the hour leading up the eruption.
Michael Schade was among the people who visited the island on Monday. Posting a video of a huge plume of smoke, he said he and his family were on a boat waiting to leave the island when they saw the eruption.
“Boat ride home tending to people our boat rescued was indescribable,” he said.
New Zealand prime minister Jacinda Ahern said there was a “number of tourists on or around the island” at the time of the incident.
“I know there will be a huge amount of concern and anxiety for those who had loved ones on or around the island at the time,” she said. “I can assure them that police are doing everything they can.”
Meanwhile, it is believed that a tour party from a cruise ship may have been caught up in the incident.
Kevin O’Sullivan, chief executive of the New Zealand Cruise Association, told Reuters, “We believe there is a tour party from Ovation of the Seas involved in the White Island eruption. We have no further details at the moment.”
Ovation of the Seas is a 16-deck cruise ship owned by Royal Caribbean Cruises. It can take nearly 5,000 passengers and has a crew of about 1,500. It is docked at Tauranga, on the North Island, on a cruise that began in Sydney on Dec 3.
“We can confirm that a number of our guests were touring the island today,” a company representative said in an emailed statement. “We do not have any additional details to share at this time.”
The White Island volcano is one of New Zealand’s most active.
The White Island volcano’s last fatal eruption was in 1914, when it killed 12 sulphur miners. There was a short-lived eruption in April 2016.