The Tangiwai disaster was the worst rail accident in New Zealand history. It occurred on December 24, 1953 when the overnight express train between Wellington and Auckland passed over the Tangiwai Railway Bridge. The bridge, which had just minutes earlier been weakened by a lahar from Mount Ruapehu, collapsed sending the train into the Whangaehu River.
Of the 285 people on the train that night, 134 survived and 151 died. Of those that died 20 bodies were never recovered; it is believed they were washed out to sea.
Evidence given at the commission of enquiry into the disaster revealed that the midstream piers of the railway bridge had been undermined by previous sudden floods, from as early as 1925. While large concrete blocks, weighing several tons, had been placed around the footings of these piers and the space between the blocks and the piers backfilled with gravel, the lahar was strong enough to sweep these away.
The cause of the lahar that led to the disaster was believed to be the overtopping and sudden collapse of a natural volcanic ash dam that had blocked the outlet of the crater lake on top of Mount Ruapehu. When that dam collapsed, the water from the lake mixed with the material from the ash dam and rushed down the mountainside in a flash flood known as a lahar. Until this disaster, the danger posed by lahars from Mount Ruapehu was only appreciated by a few scientists. A lahar warning system was subsequently installed to alert railway controllers of high river flows.