KiwiRail had to replace the brakes on all of its China-made freight wagons last year after tests found they were substandard.
The 500 wagons, worth a total of $49 million, failed to stop within the required 650 metres when fully loaded during initial tests before they entered service, according to documents obtained by Radio New Zealand.
The company replaced the brake blocks with a slightly higher-friction version.
KiwiRail said that was the quickest and most cost-effective solution, with the costs borne by the manufacturer as they were under warranty.
Labour's Clare Curran says it's appalling that the brakes weren't up to scratch.
"To have 500 wagons brought into the country and every single one of them to have to have its brake pads replaced is fairly extraordinary, and indicates poor quality workmanship, and not being fit for purpose for New Zealand conditions," she told Radio New Zealand.
"You've got to ask yourself how this managed to happen and who's accountable."
Ms Curran was critical last year when KiwiRail axed 44 jobs at its Hillside Workshops in her Dunedin South electorate, and sent the wagon work to China, where it could be done cheaper.