Mud flowed down Mt Ruapehu's slopes but no lava was produced

The eruption at Mt Ruapehu triggered two mud flows, while pilots spotted ash clouds 4km (2.5 miles) above its peak.

Around 50 people were evacuated from ski lodges on the mountain, while roads and train lines were briefly closed.

The climber was hurt when rocks crashed through the roof of a mountain hut where he and three others were staying.

William Pike's legs were crushed by a boulder, trapping him.

The 22-year-old teacher was taken to hospital in Waikato, where he was said to be in a serious condition.

Two other workers at a ski area suffered minor injuries, a council official said.

Impact craters

Vulcanologist Brad Scott flew over the site after the short eruption on Tuesday evening.

Mt Ruapehu is one of New Zealand's most active volcanoes

"The top of the mountain is black, covered in crater-lake mud and ash," he told Radio New Zealand.

Craters caused by rocks and debris could be seen up to 1.5km (almost one mile) from the summit, he said.

The eruption was caused by the build-up of steam under the crater lake rather than new lava flowing into the volcano, he said.

Experts have not ruled out further eruptions and the mountain's ski area remains closed, the Dominion Post reported.

At 2,797 metres (9177 feet), Ruapehu is the North Island's highest mountain and one of New Zealand's most active volcanoes.

An eruption in 1995 sent boulders the size of cars flying through the air.