"A number of people are reportedly injured and are being transported to shore".
White Island sits about 50 kilometers (30 miles) offshore from mainland New Zealand.
Police said a number of people remain trapped but they have not had contact with the island.
According to GeoNet - a partnership between the Earthquake Commission (EQC), GNS Science, and Land Information New Zealand - the Volcanic Alert Level has been raised to Level 4 following the eruption.
A volcano has erupted on New Zealand's White Island leaving one dead and many unaccounted for.
Local reports suggested around 20 people could have been on the island.
"'While it was initially believed there were about 100 people on or near the island at the time of the eruption, we now believe there were fewer than 50", says a police spokesperson.
It follows the eruption at around 2:10pm local time (1:10am Irish time) on Monday.
An island volcano has erupted while tourists were visiting in New Zealand's Bay of Plenty.
What looks like a group of people down the crater was captured on the White Island Crater Rim webcam shortly before the explosions were reported at the island.
"We have significant concern for people that may have been on the island", he said.
Volcanologists said the ash plume shot 12,000 feet (3,658 m) into the air.
According to the GeoNet website, more than 10,000 people visit the island ever year.
"At this stage it is too unsafe for Police and rescue services to go to the island, however we continue to assess the conditions which would allow us to go onto the island".
One tourist has died after a volcano has erupted on New Zealand's White Island.
What put people in danger was not the eruption itself but rather their proximity to the vent.
Ken Gledhill from GeoNet, the country's geological hazard information centre, said: "It's not a particularly big eruption, nearly like a throat-clearing eruption, and that's probably why material won't make it to the mainland".