WHAKATANE (REUTERS) - New Zealand said on Tuesday (Dec 10) that eight people were missing, presumed dead, a day after a volcano unexpectedly erupted off the coast of New Zealand's North Island, killing at least five people and injuring more than 30.

Prime Minister Jacinda Ardern told reporters in Whakatane, a town near the volcanic White Island tourist attraction, that aerial reconnaissance flights had shown no signs of life.

"It's now clear that there were two groups on the island - those who were able to be evacuated and those who were close to the eruption," Ardern told reporters.

New Zealanders and tourists from Australia, the United States, Britain, China and Malaysia were among the missing and injured, she said, adding that there were two explosions in quick succession. No further details were given.

"To those who have lost or are missing family and friends, we share in your unfathomable grief in this moment at time and in your sorrow," Ardern said.

Waikato Police Superintendent Bruce Bird said 47 people visited the island on Monday - five were confirmed to have been killed and eight were missing.

Some 31 were in hospital and three had been discharged.

Police said they did not expect to find any more survivors from the eruption, which spewed a plume of ash thousands of feet into the air. Many of the injured were in critical condition, most from burns, Ardern said.

White Island is about 50km (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 ft (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." Bird said rescue services were working to return to the island and were relying on advice from scientific and technical experts meeting in Wellington on Monday.

"We will only go to the island if it is safe for our people," said Bird.

Many day tours visit the island regularly. One from a 16-deck cruise liner, Ovation of the Seas, was there at the time.

Ardern said helicopters made a deliberate decision to fly to the island to rescue survivors immediately after the eruption.

"I want to acknowledge the courageous decision made by first responders and those pilots who in the immediate rescue effort made an incredibly brave decision under extraordinarily dangerous circumstances in an attempt to get people out," said Ardern.

"As a result of their efforts a number of people were rescued from the island."

'THIS IS NOT A JOKE'

Janet Urey, 61, a nurse from Richmond, Virginia, said her son Matthew, 36, was injured in the eruption while on honeymoon.

"The phone rang at midnight. Then I heard a voicemail come on. It was my son. He said, Mom...this is not a joke. A volcano erupted while we were on the island. We're at the hospital with severe burns." She has been frustrated by the lack of information from the cruise ship he was on and from authorities.

"I have not heard a word from the cruise people. I just want the word out there. I'm not really happy with how this has been handled," she added.

A crater rim camera owned and operated by New Zealand science agency GeoNet showed groups of people walking toward and away from the rim inside the crater, from which white vapour constantly billows, in the hour leading up to the eruption.

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."

Geological hazard tracker GeoNet raised the alert level for the White Island volcano in November due to an increase in volcanic activity.

The volcano's last fatal eruption was in 1914, when it killed 12 sulphur 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 per cent is under the sea, making the massive volcanic structure the largest in New Zealand.