Police say there are 'no signs of life' on Whakaari/White Island, adding they believe anyone who could have been taken from the island alive was rescued at the time of Monday's evacuation.

"The Police Eagle helicopter, rescue helicopter, and NZDF aircraft have undertaken a number of aerial reconnaissance flights over the island since the eruption," police said in a statement at 12.12am.

"No signs of life have been seen at any point.

"Police believe that anyone who could have been taken from the island alive was rescued at the time of the evacuation.

"Based on the information we have, we do not believe there are any survivors on the island."

Police are working urgently to confirm the exact number of those who have died, further to the five confirmed deceased already, the statement said.

"As part of the recovery a NZDF ship will approach the perimeter of the island at first light to deploy drones and observational equipment to further assess the environment.

"Police continue to receive information and advice from GeoNet experts to support the recovery operation.

"The Police Disaster Identification (DVI) team are assembling in Whakatane to await deployment.

"Both New Zealanders and overseas tourists are believed to involved, and a number were from the Ovation of the Seas cruise ship.

"The Police 105 number can be used by members of the public to submit information regarding friends or family who might have been visiting White Island during the eruption.

"People from overseas can call +64 9105 105.

"They can also use the online form at the Police website.

"At the request of New Zealand Police, New Zealand Red Cross has activated the Family Links website for people wanting to register themselves as safe or register an inquiry about a loved one.

"If you are worried about a friend or family member following the White Island eruption, first contact them as you normally would.

"If you cannot make contact, you can register them through this website: https://familylinks.icrc.org/new-zealand/en/Pages/Home.aspx

"The friends and family of those involved remain at front of mind for Police.

"Support is being put in place and Police are working to provide them with information as it becomes available.

Former Whakatane mayor Tony Bonne says one of the people killed was an experienced guide for White Island Tours - "a young energetic man who's lost his life".

White Island Tours' chairman Paul Quinn said the company was deeply saddened following the significant eruption.

"Devastation is an understatement. This is a terrible tragedy and our thoughts and prayers are with everyone who has been impacted."

He said the company was currently assisting police and Civil Defence with the official emergency response.

"We acknowledge the considerable efforts from Police and Civil Defence and will continue to do whatever is necessary throughout the rescue operation."

"Our immediate focus is on supporting our staff, manuhiri and respective whānau, who have been significantly impacted and are showing immense strength and courage."

A number of the tourists on the island when the eruption happened were from the Ovation of the Seas cruise ship.

Police have not confirmed nationalities and do not know if there are people on the island alive tonight. The five people dead are people who were taken off the island today. The other 18 rescued all have injuries to some degree.

National Operation Commander Deputy Commissioner John Tims told a press conference on Monday afternoon a number of people have burns as a result from the eruption and had been taken to hospital.

He said further eruptions are possible.

Those dead are of a "range of nationalities," he said.

"The experts that we've spoken to have said it is unsafe for us to go on that island. The island is unstable ... the physical environment is unsafe for us to return to the island. It is important that we consider the health and safety of those that will return to the island."

Tims told an earlier press conference this afternoon: "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."

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

Tims told an earlier press conference it was believed fewer than 50 people were on or near it at the time of the eruption.

There were a number of tourists on or around the island at the time, Jacinda Ardern said in an earlier press conference.

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

GNS is assessing the situation at the moment and is unsure if cameras on the island are still functioning.

Dr Ken Gledhill, from GNS, said "it's not a particularly big eruption, almost like a throat-clearing eruption, and that's probably why material won't make it to the mainland".

Smoke from the eruption went around 12,000m in the air, he said.

He said it has quietened down, but added he cannot be certain there will not be another eruption in the next 24 hours.

They do not believe that all of those injured by the eruption are from the cruise ship.

Earlier this afternoon, police said: "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."

"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."

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.

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.

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.

People can be seen inside the crater just before the eruption. Photo / GeoNet

Port company chief executive Mark Cairns says he understands the majority of those injured in the eruption are from the ship, the world's largest cruise vessel.

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.

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.

Eruption on White Island today. Photo / Supplied

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.

The eruption as seen from a GeoNet camera on the island.

"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.

"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.

Massive plumes of smoke from Whakaari/White Island. Photo / via zurraspismo on Twitter

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.