The New Zealand Transport Agency is refusing to say what it has found out about the cause of hugely costly cracks and rutting on expressways that are just a few years old.

Photo: Supplied / NZTA

The cost of damage to the surface of the Kāpiti Expressway and the Te Rapa and Ngaruawahia sections of the Waikato Expressway has topped $80 million.

The massive bills do not stretch to long-term solutions in Waikato, covering only repairs and monitoring.

The agency has now rejected an Official Information Act request from RNZ to release any review that it has had done into the problems and their causes.

It would also not say what advice it got from road building specialists about what materials to use when it was constructing the highways.

It was "withholding of information to enable a Minister of the Crown or any department or organisation holding the information to carry out, without prejudice or disadvantage, negotiations (including commercial and industrial negotiations)", the agency said.

It was working with partners or suppliers to negotiate long-term solutions that at Waikato "addresses the mode of failure", and at Kāpiti covered the remedial treatment, it said.