Wellington - New Zealand’s Mount Tongariro volcano, which erupted for the first time in 115 years this week, remained quiet overnight but was still steaming on Wednesday and scientists said it could blow again without warning.

The eruption just before midnight on Monday (12h00 GMT) reportedly ripped open three new vents in the mountain each a kilometre wide and officials retained a volcanic eruption alert of two on a scale of five.

“As with any volcano, an eruption could occur at any time with little or no warning and there is an elevated level of risk,” the Institute of Geological and Nuclear Sciences, which monitors seismic activity, said in a statement.

Monday's eruption hurled hot rocks up to a metre wide high in the air and cast a huge ash cloud over the central North Island, disrupting air services.

The only reported damage was to a hikers' hut about a kilometre from the crater which was bombarded by flying boulders that would have been fatal had anyone been sleeping in the hut at the time, an official said.

The hut was one of four on the 19.4km Tongariro Alpine Crossing, a popular hiking trail used by thousands of walkers every year. The trail has been closed until further notice.

Nico Fournier, a volcanologist at the geological institute, was quoted in Wellington's Dominion Post as saying volcanic gas and periodic earthquakes had been recorded under the mountain since mid-July.

Tongariro had experienced only one or two shakes a year in the past decade, but scientists insisted there were no typical warning signs that an eruption was imminent.

Fournier said the quakes were centred two to seven kilometres under the mountain and had probably been triggered by magma “looking to find a way out”.

Heavy rain and low cloud continued to prevent scientists flying low over the mountain to inspect it. - Sapa-dpa