LISTEN LIVE: Police to give latest update on White Island at 9pm:

At least five people are dead after Whakaari/White Island erupted today, and "a number" of people are still unaccounted for.

National Operation Commander Deputy Commissioner John Tims told a press conference at 9.10 tonight New Zealanders and tourists are impacted.

A number of the tourists are from the Ovation of the Seas cruise ship, but police cannot confirm nationalities.

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A number of people have burns as a result from the eruption and have been taken to hospital.

Nine News Australia is reporting 24 Australian citizens were on White Island when it erupted.

Former Whakatane mayor Tony Bonne says most people are feeling numb following the White Island eruption.

He says a young, energetic man has lost his life and added his thoughts were with the people affected.

Tims earlier told the press conference: "At this stage it is too dangerous for police and rescue to go onto the island ... the island is currently covered in ash and volcanic material."

"We know the urgency to get back to [the] island."

Twenty-three people have been taken from the island. Tims was unclear about the number of people left on the island but said it was believed fewer than 50 people were on or near it at the time of the eruption.

Earlier, Prime Minister Jacinda Ardern addressed the nation.

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

"I know that there will be a huge amount of anxiety for those who had loved ones on the island at the time."

"While it was initially believed there were approximately 100 people on or near the island at the time of the eruption, we now believe there were fewer than 50," police said in a statement earlier this afternoon. "Some of those people have been transported to shore, however a number believed to be on the island are currently unaccounted for."

Emotional families of those affected are gathering at Whakatane wharf. People covered in ash can be seen arriving for treatment after being transported from rescue helicopters.

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

"Emergency services are working to ensure the safety of everyone involved, including rescue staff."

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Earlier the Prime Minister said about 100 people had been believed to on or near the island. Jacinda Arden addressed the eruption at her weekly post-Cabinet meeting.

"All our thoughts are with those affected at this stage," Ardern said.

Police were alerted of the eruption at 2.17pm, she said.

Families have been told only one family member per person is allowed beyond the police cordon.

GNS says the eruption happened at 2.11pm and sent a plume of ash 12,000ft above the vent. Ash has covered the main crater floor and continues to fall on the island.

Images from the crater rim just minutes earlier show people walking close to the crater.

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Many of the people visiting White Island today are understood to be from the cruise ship Ovation of the Seas, which berthed at the Port of Tauranga this morning.

Port company chief executive Mark Cairns says he understands the majority of those injured in the eruption are from the ship.

Cairns said the cruise ship berthed this morning and was to depart tonight.

Passengers scheduled to visit White Island would have caught buses to Whakatane to join transport to the island, he said. Wealthy passengers could have travelled by helicopter.

The ship would not now be leaving Tauranga tonight, Cairns said.

A spokeswoman for the ship's owners, Royal Caribbean, said it was not certain whether any passengers from the ship were on White Island at the time of the eruption.

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''We're not sure if our guests were involved. We're still gathering information at the moment.''

Reporter Katee Shanks, who is at Whakatane wharf, said there are emotional scenes as family have gathered and are waiting for news.

She said ambulances were arriving and boats were being moved off the wharf to accommodate the police liaison team.

A local boatie, who didn't want to be named, said he was on the water when the eruption hit and saw one of the White Island Tours boats was already heading into shore when Whakaari exploded.

The boat drove at high speed to get to the wharf and dropped off its passengers before heading back at speed towards the island.

A shelter was installed on the volcanic island in 2016 in case of an unexpected eruption like today's.

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The 2.4 tonne shipping container has been placed by the Defence Force on an old mining site to provide a natural protective barrier. It's not yet clear whether the shelter was used today.

And a man whose Facebook profile says he works at White Island Tours, Calvin Kingi, has posted a photo of the island erupting, saying:

"White Island just erupted as we left, we have our work mates and a tour still on the island, I hope they ok."

Kingi later said the boat he was on was returning to the island: "We have people in critical condition [to] help."

Tourism Bay of Plenty spokeswoman said Volcanic Helicopters and a boat have still not been accounted for.

Meanwhile, several rescue helicopters were on their way and an emergency operation centre has urgently been set up at Whakatane Hospital.

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The National Emergency Management Agency has issued a national warning and is saying it's hazardous in the vicinity of the volcano after the "moderate" eruption.

Civil Defence is telling people to stay out of the designated restricted zones, where ashfall may be a problem, and is warning people who live close to the island to stay indoors.

Long term resident Maree Reeve said it was "possibly the worst eruption she had even seen from the island".

Dan Harvey was out fishing with some friends 40km/h away from the island when he saw dark smoke erupting from it before 2.15pm.

"I looked over and saw a burst of steam coming up. There was nothing above the island at that time. It was just clear blue sky. It was unusual to see it go from nothing to steam erupting out of it.

"It was almost like a mushroom cloud ... the steam was expanding and getting bigger.

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"I knew it was odd because there was nothing above it. Often it's doing this small continuous release of steam. But it wasn't doing anything then all of a sudden this massive amount of cloud comes out."

Harvey said it's still erupting steam but it was nothing like it was when it first erupted.
He added that it happened so quickly and fast that if there were anyone on the island it wouldn't be good.

CEO of Tourism and of Plenty Kristin Dunne, said it was a "shocking" event.

"Our concern is for any visitors that have been on the island and how we can assist when they return to the mainland," she said.

The company is working closely with police and civil defence to establish more information, she said.

A volcanic eruption is hazardous in the immediate vicinity of the volcano, police said.

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"Volcanic hazards may impact on and near the volcano. Ashfall may impact areas distant from the volcano. People should stay out of designated restricted zones.

"A no-fly zone has been established.

"People living in or near the affected ashfall areas should:

1. Be aware of the potential for ashfall. Consider staying indoors. Volcanic ash could be a health hazard, especially if you suffer from breathing difficulties.

2. When indoors, close all windows and doors to limit the entry of volcanic ash.

3. If caught in volcanic ashfalls: Wear a dust mask or use a cloth handkerchief over your nose and mouth; protect your eyes by wearing goggles. Wear eyeglasses, NOT contact lenses as fine ash will get under the lens.

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4. Listen to the radio and/or TV for further assessment and updates.

5. Effects of a volcanic eruption can be experienced many kilometres from a volcano.