The Kāpiti expressway north of Wellington needs a further $25 million worth of resurfacing, just two years after opening.
Potholes and discoloured patches began appearing on the $623m road within months of it opening in February 2017.
Figures released to RNZ show the total cost of repairs is $25.2m, with taxpayers picking up more than half the bill.
Transport Agency project delivery manager Karen Boyt said water had been penetrating the seal.
"Following the opening of the expressway in February 2017, it became apparent that the surface of the road wasn't performing as expected. Investigations have shown that water has penetrated the seal layer, resulting in a discolouration of the road and changes to its texture.
"This layer is being replaced with asphaltic concrete."
Nearly 9km of the road were repaired last year, with work on a further 16km to begin next week.
Ms Boyt said that work was expected to take about six months.
Beca Infrastructure, Fletcher Construction and Higgins Contractors, which built the road, would cover the rest of the repair cost.
NZTA said the final cost of the road was not yet known, with some property settlements still under way.