For the first time in a hundred years, New Zealand's Mount Tongariro erupted Monday night, spewing an ash cloud that spans one kilometre.

A local witnessed and reported the eruption just before midnight. The witness said the eruption had created "a new hole in the side of the mountain."

The GNS said the eruption was hydrothermal-driven, and Tongariro is likely to keep an active volcanic activity for several weeks.

"An eruption could occur at Tongariro at any time with little or no warning and there is an elevated level of risk, particularly on the northern slopes and valleys of the mountain," GNS Science told APNZ.

GNS Science volcanologist Brad Scott said the agency had not detected any increased activity in the hours leading to the eruption.

"There has been no escalation coming into the eruption and we've seen no escalation post the eruption... The eruption gave us no warning ... it just crept up on us," Mr Scott said.

The central North Island mountain threw rocks and ash to as far as 1 km radius, according to Brent Crowe of the Bay of Plenty police.

It is not clear at this point whether the worst is over for Tongariro. It could be a prelude to an even bigger. It could also be the start of volcanic activity pattern, or a natural isolated event.

The volcanic alert level for Mt Tongariro has risen from 1 to 2. The aviation colour code has been raised to red, but dropped to orange Monday afternoon.

Authorities said they had not received reports of injury or damage. Still, the Ministry of Civil Defence & Emergency Management advised people within the affected areas to stay indoors.

The locals were asked to remain calm. They were also advised to make sure their water containers were safe and not contaminated by the ash fall.

Auckland University volcanologist Phil Shane told APNZ compared Monday's landmark eruption to the Caribbean volcano Montserrat. It came to life in 1995 and has continued to erupt until today.

"We don't know how long it could go. It could be a one off or it could go weeks, months years or even a decade," he said.