Five people were killed, 18 were injured and several more were left stranded after an island volcano popular with tourists erupted unexpectedly in New Zealand on Monday.

Police said some 50 people were visiting White Island when it exploded suddenly in the early afternoon -- hurling ash and rock high into the air.

Two dozen people made it off the island, five of whom have since died. The rest are being treated for injuries, including severe burns.

Nothing is yet known about a group - now estimated to number in the double digits - who are still trapped on the island.

"We're unsure of the exact numbers on there and we're unsure of their wellbeing," said deputy commissioner John Tims.

As night fell, he said volcanic activity made a rescue attempt too dangerous.

"The island is unstable, there's a danger of further eruptions, it is physically unsafe for us to return to the island"

"I've got to consider the safety of our people and emergency services staff."

The eruption occurred at 14:11, thrusting a thick plume of white ash 3.6 kilometres into the sky.

Seconds before, live camera feeds showed a group of more than a half dozen people walking on the crater floor. Then the images went black.

A "considerable number" of those caught up in the disaster are believed to be Australian, according to officials in Canberra.

As many as 30 people are also believed to be cruise passengers on a day trip from the vessel Ovation of the Seas, Kevin O'Sullivan, chief executive officer of industry body the New Zealand Cruise Association told AFP.

The ship's operator Royal Caribbean - who had billed the trip to White Island as "an unforgettable guided tour of New Zealand's most active volcano" - said "a number of our guests were touring the island" but did not confirm that number.

The ship has a capacity of around 4 000 people and set sail from Sydney last week on a 12 day voyage.

Scene of terror

Tourist Michael Schade, made it off the island just in time and was able to capture footage of the devastation.

His videos showed groups of startled tourists clustered by the shoreline, waiting to be evacuated as the ground around them smouldered, the sky filled with white debris. An ash-caked helicopter lies damaged nearby.

As his ship hurtled away, the caldera became virtually invisible, shrouded by a thick bank of ash.

Volcanic Air said they had landed a helicopter on the island shortly before the eruption carrying four visitors and one pilot. All were now accounted for.

"It had landed on the island. What happened after that we don't know, but we know that all five made it back to Whakatane on one of the tourist boats," a company spokesman told AFP.