Almost one in 10 of KiwiRail's new freight wagons built in China is already out of commission.
Forty-four of the 500 flat top wagons that arrived last year are undergoing repair or maintenance.
KiwiRail says it plans for at least 5% of the wagon fleet to be undergoing maintenance or repair and the current situation is not that far from normal.
But the Rail & Maritime Transport Union says given KiwiRail went for the cheapest option, it's not surprising there are quality issues.
General secretary Wayne Butson says union members also say the wagons have dubious welds and are difficult to load.
The wagons were bought in two orders for $49 million. Ordering the work offshore led to redundancies at KiwiRail's Hillside workshops in Dunedin.