Footage of the White Island eruption filmed by a tourist who was leaving just as disaster struck.

Police say anyone who didn't get off White Island has likely perished.

"The police Eagle helicopter, rescue helicopter, and NZDF aircraft have undertaken a number of aerial reconnaissance flights over the island since the eruption. No signs of life have been seen at any point," police said in a release at 12.10am.

Ten hours earlier, at 2.11pm, Whakaari/White Island erupted while nearly 50 people were on it. Police still don't know how many people didn't make it off the volcano, which is a popular tourist experience, but national operation Commander Deputy Commissioner John Tims conceded in a press conference at 9pm that it was in the double digits.

Only 23 people are known to have made it off the island; five were dead and the remaining 18 all had injuries to some degree.

The Defence Force was called in to provide assistance with the rescue effort, which was stalled when GeoNet advised police the island was too unsafe to go on to as it could erupt again.

Asked if people were alive on the island, Tims said in his 9pm press conference "we don't know".

"Firstly, we're unsure exactly the numbers on there, and we're unsure of their wellbeing."

MICHAEL SCHADE/SUPPLIED A boat heads towards a damaged ash-covered helicopter on Whakaari/White Island after the eruption.

A NZDF spokesperson has confirmed the New Zealand Defence Force is helping with the emergency response at White Island, sending personnel as well as the Royal New Zealand Navy ship HMNZS Wellington.

"The NZDF will be working under the direction of NZ Police but has sent a number of assets to the area to help as needed.

"A Royal New Zealand Air Force P-3K2 Orion aircraft has flown over White Island carrying out surveillance over the area, and two NH90 helicopters have flown to Whakatane to assist. Royal New Zealand Navy ship HMNZS Wellington is currently transiting from Auckland to Whakatane to offer support."

'A tragedy'

Whakaari/White Island erupted at 2.11pm on Monday, spewing plumes of smoke into the air.

Up to 100 people were initially believed to be on or near the island at the time, but police later revised the number to be fewer than 50.

There were two people in a critical condition at Auckland's Middlemore Hospital after being flown there by rescue helicopters.

MICHAEL SCHADE/TWITTER Visitors to the island are seen huddled together after the eruption.

Prime Minister Jacinda Ardern said during a press conference that she would be heading to the scene of the eruption on Monday night.

She said those missing and injured were both New Zealand and foreign tourists.

Australian Prime Minister Scott Morrison tweeted saying Australians were involved.

"A terrible tragedy is unfolding in NZ after the volcano eruption on Whakaari/White Island. Australians have been caught up in this terrible event and we are working to determine their wellbeing."

Morrison had phoned Ardern to offer "our full support with whatever they need". The Ministry of Foreign Affairs has set up their systems to respond to concern from those abroad.

MARYLOU BRUYS White Island eruption as seen from the mainland.

Earlier Tims said it remained unclear how many were still missing - but it could be around 27.

Meanwhile, Deputy Prime Minister Winston Peters also tweeted his condolences.

"Deep sympathy to all those grieving due to the news coming from White Island."

Patients arrive

People covered in ash could be seen arriving for treatment after being transported from rescue helicopters.

It is understood passengers from the Ovation of the Seas cruise ship, currently visiting Tauranga, were among those visiting White Island at the time of the eruption.

ONE NEWS Police cordoned off Whakatāne's Muritai Drive, to give emergency crew room to receive the injured.

A spokesperson for Royal Carribean Cruises, Annie Standen, would not confirm if passengers were involved but said a representative from Ovation of the Seas was at the cordon.

She was was not able to confirm whether or how many passengers were injured.

"At this stage we are still gathering information."

A helicopter from Volcanic Air was also on the island, but the Rotorua company said the pilot and four passengers were unharmed and returned to the mainland via boat.

Don Richards, owner of the River to Sea BnB, said he was located within the cordon on Muriwai Drive and could see steam coming off the island and fleets of ambulances assembling at the wharf and helicopters landing.

He saw two White Island Tour boats coming in from the island, the second boat came in at speed about 4pm and "was covered in ash".

Some of the passengers off the boats appeared to be walking, some were sat wrapped in emergency blankets, while others were being carried in on stretchers.

STUFF Whakaari/White Island erupted on Monday 9 at 2.30pm.

An image captured at 2.10pm from a GNS camera showed people were on the island at the time of the eruption. GeoNet has since removed the White Island cameras from its website.

GNS duty vulcanologist Geoff Kilgour expressed concern for their safety.

"Our monitoring data shows that there was a short-lived eruption that generated an ash plume to 12,000ft above the vent," he said in a statement.

"Ash has covered the main crater floor as seen in our webcam images. Ash fall appears to be confined to the island and we do not expect more than a minor amount of ash to reach East Cape in the coming hours."

GNS An image taken from White Island appears to show tourists moment before the eruption.

INSTITUTE OF GEOLOGICAL AND NUCLEAR SCIENCES LTD. The moment White Island erupted from the west Crater rim.

Eruption not unexpected

GeoNet vulcanologist Brad Scott said it was always a surprise when a volcano erupted, but it wasn't completely unexpected.

"But as you are well aware, volcanic unrest has been occurring at White Island for several weeks now, so it's not a total surprise that this has led to a total eruption and we have alluded to this possibility for some time," he said.

In October Stuff reported that Sulpur dioxide gas output and volcanic tremor levels at Whakaari/White Island are at the highest levels since 2016, possibly suggesting the volcano was entering a period where an eruption was more likely.

Sulfur dioxide (SO₂) gas at volcanoes originates from magma (molten rocks), and is measured by scientists as a way to try to get an indication of what is happening to a volcano's magma system.

He said any adverse health effects for those on the mainland were very unlikely.

"On the island however, unfortunately, there will be injuries - rocks and material thrown out from the volcano would have the possibility to kill or injure anybody nearby.

"It was a significant eruption and we are concerned for anybody who was on the island at the time."

Scott said the last time White Island erupted was in 2016, this was described as a 'minor' eruption. "As White Island is an active volcano, it can erupt and this cannot always be predicted."

GEONET A GeoNet image of the White Island eruption.

Huge plumes of smoke

A large plume of white smoke was visible from the mainland during the eruption.

Opotiki resident Maree Reeve said "I certainly haven't seen a puff like this, possibly in the whole 18 years of living here".

"You can see stuff (ash maybe) falling from the cloud."

Emergency services have cordoned off Muriwai Drive to the Whakatane Heads in the wake of the eruption.

Brook Sabin White Island, 48km offshore from Bay of Plenty, is New Zealand's most active cone volcano, built up by continuous volcanic activity during the past 150,000 years.

Ash disruption

Airlines in New Zealand said there has been no effect on flights at this stage,

however, they were monitoring the situation closely.

Air New Zealand said the eruption has not affected them and directed further queries to Airways.

Brook Sabin In October Stuff reported that Sulpur dioxide gas output and volcanic tremor levels at Whakaari/White Island are at the highest levels since 2016.

Most active volcano's history

White Island, 48km offshore from Bay of Plenty, is New Zealand's most active cone volcano, built up by continuous volcanic activity during the past 150,000 years.

With 70 per cent of the volcano under the sea, it is also the country's largest volcanic structure.

GeoNet said the longest historic eruption episode was from 1975-2000. During that time, many collapse and explosion craters developed.

For long periods active vents in the craters emitted volcanic ash.

Men once lived and worked on the island.

From the 1880s, they mined the sulphur, for many uses from gunpowder to fertiliser.

On September 21, 1914, a sudden lahar in the middle of the night swept away the mine buildings in the crater and the 11 men asleep in their beds.

The only survivor was a tabby cat, Peter, who was rescued a week later when the disaster was discovered.