Imported steel from China has found to be too weak to be used on four bridges on the new $450m Huntly bypass.

Tests revealed that 500 tonnes of steel did not comply with structural steel standards and cannot be used on that section of the $2 billion Waikato expressway, Radio NZ reported.

The steel tubes were purchased at a "very low bid", but the test certificates provided turned out to be wrong.

Contractors Fulton Hogan and HEB Construction only discovered the steel tubes were not good enough after a third lot of testing.

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Initial tests were done by the steel mill and tube manufacturer in China. Secondary testing was also undertaken in NZ on samples sent from China.

"Both lots of tests said the steel met the New Zealand standard." said RNZ.

With the third set of tests, the contractors and the New Zealand Transport Agency said they tested the tubes when they arrived and discovered the steel did not comply.

"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."

That's when the steel failed.

The tubes have been used for two bridges in the bypass but they were made safe by concrete reinforcement.

NZTA 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.

Contractors were now looking to buy replacement steel for the remaining bridges.

Under the roading contract, the cost would not fall to the taxpayer.