100 near New Zealand Volcano when it erupted

At least one person was left in a critical condition when New Zealand's White Island volcano erupted suddenly on Monday, leaving authorities scrambling to treat the injured and find those unaccounted for.

Prime Minister Jacinda Ardern said the volcanic island - also known as Whakaari -- off New Zealand's north coast had erupted while groups of tourists were visiting.

"It is very much an evolving situation and of course all our thoughts are with those affected," she told reporters in Wellington, adding that an undetermined number of people were unaccounted for.

Police said the eruption occurred about 2:11 pm local time (0111 GMT) and there had been around 50 people nearby - fewer than the 100 officials originally estimated.

"Some of those people have been transported to shore, however, a number believed to be on the island are currently unaccounted for," the force said in a statement.

"Of those transported to shore, at least one has been critically injured."

St John Ambulance said there were up to 20 people believed injured but medical teams were still en route.

"Emergency services are currently frantically trying to get these people back from the island to hospital for treatment," Whakatane mayor Judy Turner told AFP.

There were particular fears for a group of visitors seen walking on the crater floor moments before the eruption occurred.

Cameras providing a live feed from the volcano showed more than half a dozen people inside the rim before images went dark when the eruption occurred minutes later.