|A general view shows the Mount Tongariro volcano|
hidden behind clouds after its eruption at Rangipo,
in Tongariro National Park. (Marty Melville, AFP)
Volcanologists admitted the eruption took them by surprise, with no seismic activity recorded at the slumbering volcano before it rumbled back to life.
The Mount Tongariro volcano, in the middle of North Island, erupted just before midnight (12:00 Monday GMT) in the first significant activity at the site since 1897, the official monitoring body GNS Science said.
Witnesses in the area, which was used a backdrop for the Mount Doom sequences in The Lord of the Rings movies, reported "flame-like explosions and a cloud of ash coming from a new hole in the side of the mountain".
Police reported no injuries or damage from the eruption.
Civil defence said it did not result in any lava flows but sent a cloud of ash 6 100m into the atmosphere, forcing the cancellation of dozens of domestic flights.
They said late on Tuesday that the immediate threat had passed, but added "an eruption could occur at Tongariro at any time with little or no warning".
New Zealand lies on the so-called "Pacific Ring of Fire", where the Earth's tectonic plates collide, making it a hotspot for earthquakes and volcanic activity.
One of the country's deadliest disasters occurred in 1953, when debris from an eruption at Mount Ruapehu, also in the central North Island, downed a rail bridge, leading to a train derailment that claimed 151 lives.
Mount Tarawera, in the same area, erupted in 1886, with a death toll estimated at 120-150 people.