WELLINGTON, New Zealand - A mix of mud, acidic water and rocks tore down the slope of a volcano in New Zealand on Sunday, bursting through a 23-foot wall of volcanic ash and sand built up in an eruption 12 years ago.

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

A lahar that tore down the same volcano in 1953 killed 151 people when it washed away a rail bridge, plunging a passenger train into the raging torrent of liquid mud.

More than 130 feet of the tephra's wall was washed away - a dozen years after it had built up in Ruapehu's 1995 eruption, said Dr. Harry Keys, a lahar expert with the Conservation Department.

"The water was a concrete color ... it was so gray. You could feel the rocks in the water hitting the bank," he told National Radio.

There was also no threat to human life. Scientists had been able to predict the lahar's passage and the early warning system had worked as planned, Conservation Minister Chris Carter said.