The subsequent board of inquiry found that the accident was caused by the failure of the tephra dam holding back nearby Mount Ruapehu's crater lake, creating a rapid mudflow (lahar) in the Whangaehu River which destroyed one of the bridge piers at Tangiwai only minutes before the train reached the bridge.

With 285 passengers and crew,[1] the train was stated by a witness—the station agent at Tangiwai railway station—to have passed through on time at 10:20 pm at about 40 miles per hour (65 km/h).

[2] The specified maximum track speed between Hīhītahi (to the south of Tangiwai) and Ohakune (to the north) at the time was 50 miles per hour (80 km/h).

Approaching the bridge over the Whangaehu River at Tangiwai, in response either to passerby Cyril Ellis standing by the track and waving a torch or upon seeing the condition of the bridge, driver Charles Parker shut off steam and initiated an emergency air brake application, while his fireman, Lance Redman, shut off the oil supply valve for the fire.

However, these actions could not stop the train from running onto the bridge, which collapsed, sending the locomotive, its tender and five second-class carriages into the river.

[1] The leading first-class carriage teetered on the edge of the bridge before its coupling to the rest of the train snapped and it, too, rolled into the river.

[4] Among the dead was Nerissa Love, the fiancée of cricketer Bob Blair, who was playing in a test match in South Africa at the time.

[7] Prime Minister Sidney Holland arrived at Tangiwai early on Christmas morning after a high-speed drive down from Auckland.

Inglis and a passing traveller, Arthur Dewar Bell, both received the British Empire Medal for actions that saved fifteen lives.

Queen Elizabeth II and the Duke of Edinburgh were visiting New Zealand on their first royal tour when the disaster occurred.

[11] The board found that a lahar from Mount Ruapehu had removed the fourth pier a few minutes before the train started to cross the bridge.

If the level indicates a significant risk, the control centre sets the signals on either side of the Tangiwai bridge to danger and warns trains in the area to stay clear by radio.

The early warning systems worked as planned, stopping trains and motorists at Tangiwai before the lahar hit.

[19] The 2002 documentary The Truth About Tangiwai, directed by New Zealand filmmaker David Sims, examines events surrounding the tragedy.