The Tangiwai disaster involved the collapse of a railway bridge on the North Island of New Zealand due to a lahar, which is a destructive volcanic mudflow. As the Whangaehu River flooded after heavy rainfall, the lahar swept down from the nearby Mount Ruapehu, damaging the bridge. A passenger train was approaching the bridge when it gave way. Despite a warning that had been sent to the train, the operator was unable to stop in time, resulting in the train plunging into the river below. This catastrophic incident took place on Christmas Eve, shocking the nation.

The accident claimed the lives of 151 individuals, making it one of the most lethal rail disasters in New Zealand's history. Victims included passengers and crew members, and the disaster prompted an outpouring of grief across the country. Rescue efforts were swift, but they were hampered by the difficult weather conditions and the unstable landscape. The aftermath saw numerous families grappling with the loss of loved ones and an investigation into the cause of the disaster, which found that better warnings could have potentially saved lives.

The government and railway authorities undertook massive reviews on railway infrastructure throughout the country. This included rigorous assessments of bridges and other critical structures at risk from natural disasters, marking the beginning of substantial improvements in safety regulations and emergency preparedness for such sudden events. The Tangiwai tragedy remains a pivotal moment in New Zealand's transport history.