The 90-year-old bridge, Bridge 57, was constructed in 1936 and is one of about 1600 bridges KiwiRail manages across the rail network.
It crosses the Rangitata, one of Canterbury’s largest braided rivers, and is an important rail transport link for much of the South Island, with about 45 freight trains crossing it each week.
In April 2024, amid heavy rainfall and flooding, floodwater scoured away the riverbed around one of the massive 120-tonne concrete piers that supports the 610m-long bridge, causing the pier to sink, then collapse.
KiwiRail only found out when a member of the public raised the alarm.
Chief investigator Louise Cook said it was only down to “sheer timing luck” the outcome was not far worse.
“If a train had traversed that bridge it’s virtually certain the middle support of the bridge would have been lost completely, the train would have derailed entering the river and of course the consequences of that are catastrophic.
“It’s very likely there’d have been injury to the train crew, the loco[motive] and wagons damaged, possibly environmental damage to the river and of course further structural damage to the bridge itself.”
TAIC chief investigator of accidents, Louise Cook. Photo / RNZ, Baz Macdonald
TAIC found KiwiRail was aware of the risk of scouring at the bridge and of the heavy rain, river flow and flood surge expected to reach the rail bridge on the morning of April 12.
Multiple conference calls were held with members of KiwiRail’s infrastructure and train control groups, and the company was aware the river flow exceeded KiwiRail’s own severe weather red-alert overnight.
The peak flood flow was expected to hit the bridge about 8am on the morning of April 12.
A northbound freight train crossed the bridge just after 6am, and another freight train went across at 8.30am.
An inspection was carried out at 8.53am, which included crossing the bridge in a hi-rail vehicle (which can drive on both rail and road) and taking photos of the river level, but did not inspect the bridge’s piers or the debris mounting at some of the piers.
The inspection was completed by 9.41am, and the inspector reported the line was safe to remain open.
The commission estimated the collapse happened around 10.30am.
At 11.28am, a member of the public contacted KiwiRail to alert them to the partial collapse of the bridge.
The next train was scheduled around 2.30pm.
The commission estimated the collapse happened around 10.30am. Photo / KiwiRail
TAIC found inspections of the bridge had not met KiwiRail’s standards, creating a missed opportunity to identify and respond to the flood risk.
Investigators found KiwiRail conducted two of the three types of inspections required – general and detailed – though noted the information reported was inconsistent.
However, KiwiRail could not provide evidence of any engineering inspections of the bridge, and only provided evidence of one engineering inspection being completed across the entire rail network between July 2016 and February 2024, investigators said.
TAIC also noted KiwiRail’s lack of a formal audit process.
It found that if an engineering inspection had been carried out it was likely the risk of the pier failing because of the river’s scour would have been better understood, and if an audit system in line with KiwiRail’s own quality management system had been put in place, the inconsistencies in its reporting would have been identified and improved.
TAIC also concluded it was very likely the river was above the red-alert level when the two freight trains and the inspector crossed it, presenting a “significant and unmitigated risk” to the train drivers and track inspector.
Other issues identified by TAIC included the lack of a specific plan to deal with the identified scour risk, which can be particularly acute in braided rivers as they shift and move, constantly changing the pressure points, and KiwiRail’s failure to have specific responses to the risks from adverse weather events.
The report said KiwiRail was aware of the severe weather conditions and had relayed the information that the river flow had exceeded the red-alert level in the early hours of April 12.
According to its own procedures, KiwiRail should have ceased operations, but the line remained open.
The commission was particularly concerned that repeated inquiries into derailments in similar circumstances had produced similar recommendations, but the same issues were coming up time and time again, Cook said.
The report listed four previous inquiries into derailments involving heavy rain, flooding or changing river conditions affecting rail infrastructure – in Kāpiti in August 2021, Hunterville in December 2021, Te Puke in January 2023 and a track subsidence beside the Rangitata River in 2002 that saw two locomotives and five wagons derail, plunging into the river.
“The Commission has real concern this inquiry isn’t just limited to Bridge 57 over the Rangitata River at all,” Cook said.
“It’s a far more systemic concern about the way KiwiRail is managing the infrastructure and that the management is risk-based and how important that is, particularly if it’s infrastructure that’s vulnerable to the increasing severe weather events we’re seeing.”
TAIC made three recommendations:
That KiwiRail ensure its asset inspections and audit procedures are in compliance with its own quality management system and standards particularly for riverbed monitoringThat it develop and document a risk-based management and maintenance system for flood and scour risk to rail bridges across its networkThat it reviews the Trigger Action Response Plans (part of its severe weather response) for high-risk assets.
KiwiRail chief infrastructure officer Siva Sivapakkiam said it accepted TAIC’s findings and recommendations, and recognised how serious the risk was.
KiwiRail chief infrastructure officer Siva Sivapakkiam. Photo / RNZ, Rayssa Almeida
He said KiwiRail had implemented a drone survey to assess whether changes in river flow or morphology could compromise bridge performance in future flood events, and had reviewed and improved the company’s bridge inspection regime.
KiwiRail released its first dedicated Climate Resilience Plan in 2025, which acknowledged New Zealand’s climate was changing in ways that affected its ability to move people and freight around the country, and was making changes to how it managed assets and services, Sivapakkiam said.
“We are committed to continuously improving the safety and resilience of our infrastructure and we are working hard to ensure this type of incident does not occur.”
A Metlink passenger train on Wellington’s Johnsonville line derailed with nine people on board on the weekend, injuring six people.
A TAIC investigation is underway, but could take between one to two years.
Cook told reporters at the time it was too early to determine if the heavy rain was a factor.