It's a tourist destination and police believe 50 people were on or near the island when the eruption took place.
The massive ash plumes have meant no emergency services have been able to reach the island.
Where is the volcano?
It's in the Bay of Plenty, east of Auckland, on the North Island. The volcano is about 50km from the shore.
Tourists can be seen on a trail near the volcano's crater. Source: GeoNet
Who's invoved?
Police have reported of the 23 people ferried back to shore, five people have died, and others have serious injuries and burns.
But what we don't know is the state of people trapped on the island; there could be as many as 27 people and we don't know how they are.
Are there Australians involved?
Yes. Prime Minister Scott Morrison says Australians are "caught up" in the tragedy - but we don't know if they're among the five dead, those injured, or on the island.
A cruise ship, the Ovation of the Seas, which originated in Sydney and was docked in nearby Tauranga, had a day boat of passengers visiting the island.
When will we know more?
Police and emergency services are currently taking expert advice on when it may be safe to reach the island.
They will attempt to do so as soon as they can.
What do I do if I might know someone involved?
The Department of Foreign Affairs and Trade has a hotline for consular support.
It is 1300 555 135, or if you're outside Australia, +61 2 6261 3305.
