A video shows the dramatic moment tourists on a boat watched Whakaari/White Island erupt, moments after they were standing in its crater.

People on the boat can be heard shouting "go inside, go inside!" as the massive plume of steam and debris blasts out of the crater of the Bay of Plenty volcano.

The footage, shot by Allessandro Kauffmann and posted on Instagram, captures the moments before the eruption – people smiling and enjoying the boat ride.

The boat passengers rush to get cover. Photo / Allessandro Kauffmann
The boat passengers rush to get cover. Photo / Allessandro Kauffmann

And then the volcano comes alive.

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Kauffmann, from Brazil, posted on Instagram: "Some people have extensive burns on their bodies. Two tours on the volcano. Ours was the first. The other one right after. We left the island and wasn't even five minutes before it erupted. This other tour that arrived after couldn't leave in time.

"Some people have serious burns. We had to stay to help those people who were on the island. The boat from this other tour was covered in ash from the volcano. Very tense talking about this. We just have to hope that all is as well as can be."

Police confirmed at least five people were dead and many more were unaccounted for, and others have critical injuries.

Authorities expect the toll to rise.

An estimated 50 people were on or near the island and of that number 23 were evacuated.

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100 people on Whakaari/White Island when it erupted

Another tourist near the island, Michael Schade, posted dramatic footage and images of the moment the volcano erupted.

"My god, White Island volcano in New Zealand erupted today for first time since 2001," Schade posted on Twitter.

"My family and I had gotten off it 20 minutes before, were waiting at our boat about to leave when we saw it. Boat ride home tending to people our boat rescued was indescribable."

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"This is so hard to believe. Our whole tour group were literally standing at the edge of the main crater not 30 minutes before. My thoughts with the families of those currently unaccounted for, the people recovering now, and especially the rescue workers… "

An active police search and operation is under way, Prime Minister Jacinda Ardern told the conference.

There were a number of tourists on or around the island at the time, Jacinda Ardern said.

Brazilian tourist Allessandro Kauffmann describes scenes of badly burned members of tour groups as they tried to flee the island. Video / @allessandrokauffmann

Cameras providing a live feed from the volcano showed more than half a dozen people walking inside the rim at 2.10pm, before images went dark when the eruption occurred minutes later.

Other dramatic video taken from a boat shows the sky covered in ash.

Twelve people were killed on the island in 1914 when it was being mined for sulfur. Part of a crater wall collapsed and a landslide destroyed the miners' village and the mine itself.

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The remains of buildings from another mining enterprise in the 1920s are now a tourist attraction.

The island became a private scenic reserve in 1953, and daily tours allow more than 10,000 people to visit the volcano every year.