MOUNT TONGARIRO: A New Zealand volcano has erupted unexpectedly after lying dormant for more than a century, spewing an ash plume that disrupted flights and closed highways, officials said Tuesday.

The Mount Tongariro volcano, in the middle of North Island, erupted just before midnight(local time) in the first significant activity at the site since 1897, the official monitoring body GNS Science said.

Police said witnesses reported “flame-like explosions and a cloud of ash coming from a new hole in the side of the mountain”.

“There were rocks being thrown out. It was like thunder and lightning and fireworks. It was spectacular,” local resident David Bennett told Fairfax Media.

Police said there were no reports of injuries or damage, although they were checking wilderness huts in the remote area which are used by hikers for shelter.

Civil Defence said the eruption did not result in any lava flows but sent a cloud of ash 20,000 feet into the atmosphere, forcing the cancellation of dozens of domestic flights.

While Civil Defence officials did not order any evacuations, they advised residents beneath the cloud to stay indoors with windows and doors sealed.

Volcanologists admitted the eruption took them by surprise, with no seismic activity recorded at the slumbering volcano before it rumbled back to life.

With GNS Science warning further eruptions could occur at any time, Prime Minister John Key said authorities were watching the situation closely.

“Civil Defence and others will continue to monitor the situation and if we believe it presents more significant risks then obviously we'll make sure that everyone's well and truly notified of that,” he told reporters.

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.

Air New Zealand said the Mount Tongariro eruption had affected domestic flights to Gisborne, Rotorua, Taupo, Napier and Palmerston North.

“We will not fly through ash and are constantly taking guidance from the CAA (Civil Aviation Authority) and the MetService to ensure we can continue to carry passengers where safe routes and altitudes are available,” the airline said in a statement.

The CAA said international flights were not expected to be disrupted, as they cruised above 20,000 feet.

Police said several highways in North Island were initially closed due to poor visibility and reopened Tuesday morning.

GNS Science volcanologist Michael Rosenberg said volcanic activity could continue for weeks.

“This eruption caught us by surprise. We've been monitoring the area after earthquakes, but we didn't expect this,” he tld TVNZ.

“This might just be a quiet period and we should expect it to start again at any time. So we are watching things very closely.”