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A train passes over a bridge on the Whangaehu River at the scene of the historic Tangiwai rail disaster after a mud flow from the crater lake of Mount Ruapehu on New Zealand's North Island on Sunday. ((NZPA/Stephen Barker/Associated Press))

A potentially lethal mix of mud, acidic water and rocks tore down the slope of New Zealand's Mount Ruapehu on Sunday, emergency officials said.

There was no immediate threat to life and no reported injuries.

The mud flow, also known as a lahar, broke through a rubble wall atop the mountain's crater lake about 10:47 a.m. local timeSunday, triggering an early-warning alarm, Horizon regional council said.

"It didn't take long until all the boulders justdisappeared and you could hear the rumbling as they moved with the volume of water," an unidentified local resident said.

On Dec. 24, 1953,alahar tore down the same volcano, killing 151 people when it washed away a rail bridge and a passenger train plunged into the raging torrent of liquid mud. It became known as the Tangiwai disaster.

Police and civil defence workers immediately closed roads and the nation's main trunk rail track near the southern base of the mountain on New Zealand's North Island.

A spokesman for New Zealand's railway system, Ontrack, said they had early warning of the lahar.

"We've then had everybody clear, had trains clear to make sure that nobody was placed at risk," William Peet said.

The island's main north-south highway, about 50 kilometres from the mountain's base, also was closed, while two passenger trains with 200 people on board were halted some distance from the mountain.

New Zealand Conservation Minister Chris Carter said the early-warning system had saved lives.

"We now have a system that works, that we'll need in the future because it's a very dynamic landscape on Mount Ruapehu, and we've had 45 lahars since the Tangiwai disaster."