Sixteen hundred tonnes of steel from China has been found to be too weak for four bridges on the $450 million Huntly bypass that forms part of the $2 billion Waikato Expressway.
Contractors building the 'Road of National Significance' chose a very low bid for the steel tubes.
But the test certificates for them have turned out to be wrong, and now an expensive fix-up job is under way.
The contractors, Fulton Hogan and HEB Construction, have admitted to RNZ News the steel tubes were not good enough. They did not comply with standards for structural steel, which for bridges were very high as they must resist impacts, heavy loads and low temperatures.
It was only after a third lot of testing that the contractors found out. The first tests were done in China by the steel mill and the tube manufacturer; it is understood the second tests were done in New Zealand on samples sent here from China.
Both lots of tests said the steel met the New Zealand standard.
As for the third testing, there are two versions of events. The contractors and the New Zealand Transport Agency say that, following established quality control processes, they tested the tubes after they arrived and immediately found out the steel was no good.
But RNZ News has been told it was only when workers began pounding the tubes into the ground, and the steel ballooned on the ends, that tests were done by an accredited laboratory.
The steel failed in this third lot of tests.
The contractors said the weak steel was "identified on delivery to site, when the piles were assessed against our designers' specification through our standard quality assurance process".
They have used some of the tubes anyway, for two bridges in the Huntly bypass, but say they were made safe by concrete reinforcement. They say they did not have time to do anything else.
The contractors are now looking to buy replacement steel tubes for the two other bridges.
The road-building contract means none of this cost falls on the taxpayer - it all falls on the contractors. There are probably commercial warranties in place, but it could also be that the contractors now pursue the supplier of the tubes.
RNZ News is trying to identify the supplier or intermediary; the contractors would not disclose whether it was a New Zealand supplier, or if they had their own supply chain in China.
The New Zealand Transport Agency has declined to talk about what assurances it was seeking from the contractors to ensure this did not happen again.
In a statement, the agency said there was no safety issue with the four bridges, and that "steel pipe piles and reinforced concrete piles do exactly the same job and last exactly the same time".
It said there would be no delay in the project.
RNZ News understands the contractors went for a bid that was 30-40 percent lower than an average market price for these tubes.
This was in a world market flooded with Chinese steel, ranging from the very good to the very bad. RNZ News is working to identify the mill and manufacturer in China and what accreditation either has there to make or test steel tubes.