WELLINGTON, March 19 (Reuters) - New Zealand scientists kept watch on a volcanic crater on Monday after a natural dam burst, sending a torrent of muddy water and debris down the side of a peak used as a backdrop in the "The Lord of the Rings" films.

Up to 1.3 million cubic metres (46 million cu ft) of water and volcanic mud -- known as a lahar -- swept trees and boulders down the side of the 2,797-metre (9,177 feet) Mount Ruapehu in the country's north on Sunday.

Alerted by an early warning system, emergency services had closed roads and rail lines in the mudflow's path, and no injury or damage was reported in the isolated region.

New Zealand's Department of Conservation said scientists had flown into the crater of the volcano in the centre of the North Island and found the lake's level had fallen about 6.3 metres.

"There is some debris from the dam left, although most of it went yesterday," Department spokesman Dave Wakelin said.

A lahar in 1953 destroyed a railway bridge, causing a packed passenger train to plunge into the Whangaehu river, killing 151 people.

An eruption in 1995 created the latest natural dam of ash and pumice on a section of the crater rim, about 7 metres high and 80 metres wide, and scientists had been checking the lake as it had been steadily rising over the past few months.

While water levels had subsided, another lahar could occur at any time as a result of volcanic activity, and the crater lake would continue to be monitored, Wakelin said.

Sunday's lahar flowed down the Whangaehu river, and dispersed as it neared the sea.

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