The North Island volcano roared to live without warning on Tuesday night, sending mudflows cascading down the 9,176-foot (2,797-metre) mountain.

"Looking at our seismic instruments, it was ... from nothing to full (eruption) in the space of a minute," vulcanologist Craig Miller said. He added that such occurrences are "reasonably rare."

Civil defense officials ordered nearby skiing areas evacuated and police closed all roads in the area, including the main state highway and railway line.

Three of the injuries from the eruption were minor, but a 22-year-old teacher’s legs were crushed when a rock crashed through the roof of a mountain shelter where he had taken refuge. He lost one of the legs after being taken to an area hospital.

Mount Ruapehu has erupted five times since 1969, with one in 1995 causing widespread damage when it tossed huge quantities of ash and rocks up to the size of large cars from its crater.

In June, a mixture of mud, water and rocks rushed down the slope of Ruapehu after it burst through a 23-foot wall of volcanic ash and sand built up during that previous eruption.

Live Images of New Zealand Volcanoes

Photo: Institute of Geological and Nuclear Sciences Ltd.