New Zealand police say they do not expect to find any more survivors at the site of a volcanic eruption in which five people are confirmed dead and eight are unaccounted for.

The volcano erupted off the east coast of New Zealand's North Island, spewing a plume of ash thousands of metres into the air.

The dead were "five of those that were rescued from the island earlier in the day," deputy police commissioner John Tims told reporters, adding that another 18 people were being treated for injuries, including some with severe burns.

He said there had been no contact with another group of at least ten people who remained on the island after the eruption, revealing "we're unsure of their wellbeing".

Twenty-three people, some of them believed to be tourists from the Ovation of the Seas cruise ship, were rescued from White Island, where the volcano erupted just after 2pm local time (1am this morning Irish time), police said.

Earlier, about 100 people were feared to have been nearby.

Michael Schade, an engineering manager from San Francisco, was one of the tourists who made it off the island minutes before the eruption.

He posted dramatic footage of the eruption from the boat he was on, with smoke first engulfing the top of the crater and then the entire island.

"This is so hard to believe," Mr Schade said. "Our whole tour group were literally standing at the edge of the main crater not 30 minutes before."

The island covered in ash moments after the eruption (Pic: Michael Schade)

The White Island is about 50km from the east coast of North Island and huge plumes were visible from the mainland.

Vulcanologists said the ash plume shot over 3,600 metres into the air.

"We know that there were a number of tourists on or around the island at the time, both New Zealanders and visitors from overseas," Prime Minister Jacinda Ardern told a news conference.

"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."

"We know that there were a number of tourists on or around the island at the time, both New Zealanders and visitors from overseas," Prime Minister Jacinda Ardern told a news conference.

"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."

Chief executive of the New Zealand Cruise Association Kevin O'Sullivan said: "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 3 December.

"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."

At least one of those taken to shore was critically injured, police said, adding in a statement that a no-fly zone had been set up.

"I'm not sure if these people were on the island or near it, but there was definitely one group out there and they definitely needed medical care," said Judy Turner, the mayor of the nearby coastal town of Whakatāne.

"There were some injuries and the focus is on getting these injured people back safely and to get them to a hospital."

There seemed to be no danger for people in coastal areas farther away, she added.

The island's immediate surroundings were hazardous because of the eruption, the National Emergency Management Agency said in an statement, adding that falling ash might affect some areas.

The "short-lived eruption" threw an ash plume about 3,658 metres high, New Zealand's geoscience agency GNS Science said in a statement, but added there were no current signs of an escalation.

The White Island volcano is one of New Zealand's most active.