Newly released emails have shown the New Zealand Transport Agency knew it had imported hundreds of tonnes of poor-quality steel from China, despite public denials.

The documents, obtained by RNZ under the Official Information Act, show 600 steel rods meant to be used as "piling and structural support for bridges, flyovers and various structural elements" on the Huntly section of the Waikato Expressway failed brittleness tests and couldn't be used as intended.

They also show the rods came from a mill that doesn't usually produce export-grade steel, and were tested at a lab that doesn't have accreditation.

In 2016, it was revealed 1600 tonnes of substandard Chinese steel had forced contractors to redesign bridge on the expressway so they'd meet NZ safety standards.

At the time NZTA denied there were more problems.

"There have been no similar problems identified with steel used in other state highway projects," NZTA told RNZ, rejecting the broadcaster's requests for documents, citing commercial sensitivity.

Two years later, the Ombudsman has forced the agency to hand them over.

The 600 rods were eventually cut in half and used in a new design involving more steel and concrete. The contractors wore the cost for the new design.

"It's clear from the newly released emails that the contractors, they don't often tell the agency if they have steel or other projects that fail - they design a fix, they wear the extra cost," RNZ reporter Phil Pennington, who obtained the documents, said on Thursday.

"What that can mean is the agency is oblivious to serious problems with materials."