The cause of a 2021 train derailment near Hunterville has been revealed.

A final report from the Transport Accident Investigation Commission (TAIC) says floodwaters washed away the ballast that held the tracks in position, causing them to bend out of shape.

The accident occurred around 11pm on December 13, 2021.

According to the report, the KiwiRail freight train rounded a curve and was confronted by water flowing across the line from a flooded field.

Suspecting the train had derailed, the driver immediately reduced power and stopped the train.

Seventeen of the train’s 37 wagons and the locomotive derailed.

Chief investigator of accidents Naveen Kozhuppakalam said the report showed why people in control of transport infrastructure needed to think again about how well their structures could cope with significant rainfall events.

“All the signs are that significant rainfall is increasing in frequency and severity and is increasingly likely to happen in places people don’t expect,” he said.

 “This is one more example of a transport infrastructure risk, caused by climate change; the transport industry needs to prepare, be able to adapt and respond to the increase in frequency of unseasonal and severe weather events.”

KiwiRail’s response to the accident included revised hydrology assessments and flood modelling, new flood-monitoring gauges and larger capacity culverts.

The TAIC opens an inquiry when it believes the reported circumstances of an accident or incident have - or are likely to have - significant implications for transport safety or when the inquiry may allow the commission to make findings or recommendations to improve transport safety.