The cause

Lahars are when large amounts of ice, rock, boulders, snow, silt and water, usually sitting in a crater lake beside a volcano, burst the banks and flood though a valley or down a hill. This was the case next to Mt Ruapehu. At approximatley 10:10-15pm the lakes weakened wall collapsed and about 2 million cubic metres of  debris rushed down the river in a 6 metre high wave! Some Amateur geologists warned the the crater lakes wall would burst but were catastrophicly ignored. Also in 1925 another lahar had severly weakened the bridge which would have contributed in the bridges fall.