Passerby spotted collapsing train bridge that was still open - inquiry

An inundated Canterbury rail bridge was closed to train traffic only after a member of the public noticed one of its piers had collapsed, an investigation has found.

No train was crossing bridge 57 on the main south line over the Rangitata River when a pier gave way in April 2024, but the near-miss has prompted a Transport Accident Investigation Commission probe into how KiwiRail manages flood risks.

Water levels up to 3.5m higher than their usual in the river had scoured away riverbed material supporting the bridge piers, causing one of them to collapse.

KiwiRail had not closed the line, used primarily by freight trains, and the damage was only discovered when a passerby on the adjacent road spotted it and raised the alarm.

Commission chief investigator of accidents Louise Cook said KiwiRail had been aware the bridge was vulnerable to scour but lacked bridge-specific controls to act on that risk.

"Bridge 57 is a reminder that severe weather can quickly turn a known risk into a serious hazard. KiwiRail knew this bridge was vulnerable to scour. The problem was not just the weather. It was the lack of clear, bridge-specific controls to match that risk.”

The 610m long Rangitata rail bridge was part of the rail connection between Invercargill, Dunedin and Christchurch, with around eight freight trains crossing each weekday and five each weekend at the time of the incident.

The commission found KiwiRail's inspections had not consistently recorded riverbed profiles and that audit processes had not been picking up the gaps.

The rail operator's asset-management system ranked scour risk across the national network, but did not set out what mitigation should follow for high risk bridges.

Cook said staff sent to inspect bridges during floods had no documented thresholds telling them when conditions were too dangerous.

"For a track ganger or structures inspector who is sent to inspect a bridge during a flood, they know the river is high, they know the bridge has some level of scour risk, but they have no documented threshold that says 'above this flow rate the bridge is unsafe', or 'if you see X, close the line'," she said.

When severe weather hit, the response had relied too heavily on general procedures and individual judgment, the report found.

"You cannot leave that to guesswork when the river is rising," Cook said.

"If a bridge has a known scour risk, you need to know what to look for, what action to take, and when to stop traffic."

The commission made three recommendations to KiwiRail covering inspection and audit compliance, risk-based asset management for flood and scour risk, and reviewing trigger action response plans for high-risk infrastructure.

The lessons applied well beyond one bridge, Cook said – particularly as severe weather becomes more frequent with the changing climate.

"Rail bridges across the country face flooding, scour and other weather-related hazards. As severe weather becomes more frequent, infrastructure owners need stronger systems for individual assets, not broad rules that miss the detail."

The commission said visual inspection alone may not be enough to determine whether bridges are safe.

"Communities and commerce rely on these structures every day," Cook said.

"The job is to ensure the systems around them are strong enough to spot the risk, understand it, and act on it before something fails."

KiwiRail chief infrastructure officer Siva Sivapakkiam told RNZ it accepted the findings and recommendations.

It recognised how serious the risk was and introduced a drone survey so it could assess changes in river flow and how it may affect bridges. Bridge inspection had also been improved.

KiwiRail released its first Climate Resilience Plan in 2025.

"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."