There were no reports of injuries or damage associated with the natural phenomenon known as a "lahar", Australian Broadcasting Corp. said on its Web site.
Mt. Ruapehu is one of the most active crater lake volcanoes in the world and scientists have been expecting a significant lahar for the past few months. The 9,175-feet mountain also was the location of Middle Earth in the "Lord of the Rings" trilogy of films.
Reports and the first pictures from the scene describe a flash flood of water, mud and volcanic debris sweeping down the southern side of the mountain towards Tangiwai, scene of the 1953 rail disaster that claimed 151 lives when a lahar swept away a bridge, the broadcaster said.
Hundreds of travellers have been stranded by road closures in the central north island of New Zealand, with bad weather hampering efforts for an aerial inspection of the volcano, it said.
