Mt Ruapehu more unsettled 

Volcanic activity at Mount Ruapehu is increasing and an eruption could occur at any time, scientists warned on Tuesday.

The volcano, famed as a location in The Lord of the Rings film trilogy, last erupted on September 25 2007, spitting two metre boulders distances of up to two kilometres.

Ruapehu's elevated alert level has not been changed, but scientists said on Tuesday that activity within the mountain was greater, with high levels of gas spewing out, a warmer than average crater lake and ongoing volcanic tremors.

"The volcano remains in a status of unrest and the possibility of further activity remains. If further eruptions occur, they may occur without warning," Geological and Nuclear Sciences (GNS) said in a statement.

Last September's eruption injured a climber after a boulder crashed through the roof of a hut near the summit where a party of four climbers were staying.

The highest mountain in the North Island at 2,797 metres, Mt Ruapehu has one of the most active crater lakes in the world.

In March 2007 a mudflow, or "lahar" flowed down the side of the mountain after the crater lake overflowed, and large eruptions in 1995 and 1996 blanketed the surrounding area in ash.

GNS spokesman Brad Scott says Mt Ruapehu is tricky volcano to read but they are keeping the alert level at the lowest ranking of one.

He says the eruptions in 1995 and 1996 had a long build-up, while last year's lahar came form nowhere. Scott says the situation will be monitored closely.

In 1953 a lahar swept away a railway bridge at Tangiwai at the base of the mountain and 151 people were killed when an Auckland-to-Wellington train plunged into a river.