The volcano, located around 40km offshore of the Bay of Plenty, sent huge amounts of white smoke - most likely steam - into the air in the blast.
GeoNet agency said a moderate volcanic eruption had occurred and raised its alert level to four, on a scale where five represents a major eruption.
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The wind direction means there is a safe space for the helicopters to land at present.
"I'm not sure of the extent or nature of those injuries. emergency services are all waiting for them to be returned from the island so they can attend to them". A spokesperson said a mobile triage unit was on its way to the island.
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They warn people to consider staying indoors in affected areas.
A GNS image taken at 2.10pm appears to show people on the island, but there has been no official confirmation.
Civil defence authority warned that the situation was hazardous "in the immediate vicinity of the volcano", which is 48 kilometres off the coast of Bay of Plenty, a sunny, beachfront region popular with tourists.
New Zealand's White Island volcano erupted suddenly on Monday (Dec 9), prompting fears for a group of visitors seen walking on the crater floor moments before.