A Board of Inquiry was appointed to look into the cause of the accident; this sat in public from 26 January until 2 April and reported on 23 April 1954. The bridge had eight piers and seven spans. After the accident four piers had been damaged and five spans dislodged. 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. Their subsequent report stated; The relative positions of the 44 ft. girder (span 3) and the first car (A. 1907) dictate that the girder was removed before the passage of the first car. This girder must therefore have been removed either before the locomotive arrived on the span or during the passage of the locomotive and tender over the span. If this girder was removed during the passage of the locomotive, there is a strong inference that some part of the heavy structure of the locomotive would have hit girder No. 2. There is, however, no evidence of extensive damage to girder No. 2, and it may therefore be inferred that girder No. 3 was not removed during the passage of the locomotive across it. Span 3 was therefore carried away before the locomotive entered upon it and there is a strong inference that Pier No. 4 was carried away before the train passed over it. This would also cause span 4 to be carried away. There is also a strong inference that Pier 4 was removed by the lahar and that this caused the accident. The bridge had been designed for foreseeable flooding and for previous lahars that had different characteristics to the 1953 lahar, the forces of which had been unpredictable.