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.

The mud flow — also known as a lahar — broke through the rubble wall atop Mount Ruapehu's crater lake Sunday morning, triggering an early warning alarm, local authorities said.

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.

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The island's main north-south highway, some 30 miles from the mountain's base, also was closed and two passenger trains with 200 people on board were halted some distance from the mountain.

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.

On Sunday, millions of gallons of acidic water breached the naturally occurring wall of volcanic ash and sand known as tephra, regional council chairman Gary Murfitt said.

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.

Farmer Josh Wallace said the lahar carried rocks, mud and trees down the Whangaehu River that runs through his property.

"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 were no immediate reports of damage, apart from flooding on some farmland near the base of the mountain.

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.

In the 1953 lahar, the bodies of some of the dead were washed high into trees as the torrent spewed down river valleys to the sea, some 125 miles away. Many of the victims were never found.