Six people have been confirmed dead, with Australians believed to be among them, following the volcanic eruption on New Zealand's White Island.

It's feared that death toll will rise, with Prime Minister Scott Morrison saying there could be "worse news to come".

Here's how the tragedy unfolded.

Last week, an alert was issued for continuing 'moderate volcanic unrest'

The alert, issued by New Zealand's geological hazards agency GeoNet on Tuesday, December 3, said that moderate volcanic activity on White Island, off the east coast of the North Island, had been going on since late September but was "occurring more frequently now".

"Substantial gas, steam and mud bursts [have been] observed at the vent located at the back of the crater lake," it stated.

The volcanic alert level remained at two ("we are measuring significant changes and there is an increased eruption potential") on a scale that goes up to five ("a major volcanic eruption is in progress with impact well beyond the volcano").

"Observations and data to date suggest that the volcano may be entering a period where eruptive activity is more likely than normal," the December 3 alert said.

An explainer published by GeoNet in September said that "volcanoes can erupt at any level, and levels may not move in sequence as activity can change rapidly".

The volcano's last fatal eruption was in 1914, and killed 12 sulphur miners. And in April 2016 there was another short-lived eruption.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 3 minutes 28 seconds 3 m 28 s GNS Science explains New Zealand's volcanic alert levels

This video of the volcano was taken in the hour before the eruption

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 13 seconds 13 s Tourists were standing at the edge of the volcanic crater 30 minutes before it erupted

It was taken by tourist Michael Schade, who said the tragedy was "hard to believe".

"Our whole tour group were literally standing at the edge of the main crater not 30 minutes before," he tweeted.

Mr Schade said he and his family were waiting at their boat when they saw the eruption.

This photo was taken just seconds prior

This image was captured just before the eruption. ( Supplied: White Island Crater Rim webcam )

It was taken by a camera, set up by GeoNet, which automatically posts images every 10 minutes.

Look closely and you can see hikers in the crater, near the edge of the rim.

The photo was taken at 2:10pm Monday (local time). The eruption occurred at 2:11pm.

According to New Zealand police, there were 47 people on the tiny island when the volcano erupted.

On Tuesday morning, Mr Morrison said 24 Australians, aged between 17 and 72, were on a tour of White Island at the time.

Six of those on the island were confirmed to have died, and eight were missing.

Volcanologists said the ash plume shot 3,658 metres into the air.

Witnesses were shocked by what had happened

The tourist who took this photo says it was taken about a minute or two into the eruption. ( Twitter: Michael Schade )

At the time, ABC journalists Donna Field and John Taylor were on the cruise ship Ovation of the Seas docked at Tauranga, near White Island.

"I hiked Mount Manganui and saw what I thought was smoke from a fire. It wasn't until I was back onboard that my daughter alerted me to the eruption," Field wrote.

Guests and crew from the cruise had been on a tour to White Island at the time. Field described a sombre mood on board.

"Our minds raced. Perhaps those who opted for White Island were beside us in the queue for the surfing attraction? Maybe they were the family that blitzed trivia night?" she wrote.

Shortly after the eruption, the volcanic alert level was raised to four, but it was lowered to alert level three at 4:25pm Monday (local time).

The rescue effort began immediately

White Island Tour operators take survivors away from the scene. ( Supplied: Michael Schade )

Auckland Westpac Rescue Helicopter pilot James Tayler told 1 News that they received a call for help about 2:40pm local time.

"All we really knew was there had been an eruption at White Island and that there were potentially multiple casualties," Tayler told 1 NEWS.

Two helicopters were airborne soon after, and a third joined the rescue effort at 5:30pm. It was their job to get the injured to hospital.

"They were covered in ash, looking quite badly burned," Tayler told 1 News.

Thirty-one people (of whom 13 are Australian) are being treated in hospital, while three more have been treated and released.

The only identified fatality so far is a New Zealand tour guide.

This photo was taken from an Auckland Westpac Rescue Helicopter. ( AAP: Auckland Rescue Helicopter Trust )

There's a 50 per cent chance of another eruption

This is the latest advice from GeoNet:

"Over the next 24 hours we estimate an equal likelihood of either no eruption or a smaller/similar sized eruption that would impact the main crater floor. "There is a high level of uncertainty associated with this estimate. We also estimate the least likely scenario is a larger eruption."

NZ police say work is continuing to recover the eight people who remain unaccounted for (including working with experts given the possibility of further volcanic activity).

ABC/Reuters