Police said early on Tuesday they did not expect to find any more survivors from the volcanic eruption, which occurred on White Island on Monday at about 2:11 p.m. (0111 GMT), spewing a plume of ash thousands of feet into the air.
About 50 people, New Zealanders as well as foreign tourists, are believed to have been nearby at the time and several were seen near the rim of the crater minutes before the eruption.
Rescue services have been unable to reach White Island as it remains too dangerous.
"No signs of life have been seen at any point," the police said in their statement early on Tuesday after rescue helicopters and other aircraft had carried out a number of aerial reconnaissance flights over the island.
"Police believe that anyone who could have been taken from the island alive was rescued at the time of the evacuation."
Tour operators took some people off the island before it was declared unsafe. Twenty-three people were rescued, police said on Monday, adding that others were still on the island.
"Police (are) working urgently to confirm the exact number of those who have died..." their statement said, adding that a ship would approach the island at first light on Tuesday to further "assess the environment."
Many day tours visit the island regularly. One from a 16-deck cruise liner, Ovation of the Seas, was there at the time.
"Both New Zealanders and overseas tourists are believed to (have been) involved, and a number were from the Ovation of the Seas cruise ship," the police statement said.
St. John Ambulance said up to 20 people were believed to have been injured in the eruption, adding that a mobile triage unit was on its way.
Several people with burn injuries were brought by helicopter to Whakatane, the nearest town on the mainland.
"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. I can assure them that police are doing everything they can," Prime Minister Jacinda Ardern told a news conference after landing in Whakatane late on Monday.
Ardern was expected to give an update on the situation at a news conference set for 7 a.m. on Tuesday (1800 GMT on Monday).
"DISASTER WAITING TO HAPPEN"
Michael Schade, an engineering manager from San Francisco, was one of the tourists who made it off the island just before the eruption.
"This is so hard to believe," Schade said in a video posted on Twitter as he sped away from the island by boat. "Our whole tour group were literally standing at the edge of the main crater not 30 minutes before."
A crater rim camera owned and operated by New Zealand science agency GeoNet shows groups of people walking toward and away from the rim inside the crater, from which white vapor constantly billows, in the hour leading up to the eruption.
White Island is about 50 km (30 miles) from the east coast of North Island and huge plumes were visible from the mainland. Volcanologists said the ash plume shot 12,000 feet (3,658 m) into the air.
"White Island has been a disaster waiting to happen for many years," said Ray Cas, a professor emeritus at Monash University, in comments published by the Australian Science Media Centre.
"Having visited it twice, I have always felt that it was too dangerous to allow the daily tour groups that visit the uninhabited island volcano by boat and helicopter."
Geological hazard tracker GeoNet raised the alert level for the White Island volcano in November due to an increase in volcanic activity.
The White Island volcano's last fatal eruption was in 1914, when it killed 12 sulfur miners. There was a short-lived eruption in April 2016. Daily tours allow more than 10,000 people to visit the volcano every year.
'Whakaari', as it is known in the Maori language, is New Zealand’s most active cone volcano, built up by continuous volcanic activity over the past 150,000 years, GeoNet said.
About 70 percent is under the sea, making the massive volcanic structure the largest in New Zealand.