The scale of the problem plaguing the leaky Kāpiti expressway has been revealed.

Forty-nine kilometres of lanes are estimated to need repairs – a job that will take two years to complete on the four-lane, $630 million road that only opened to traffic early last year.

About 5km of the 18km expressway on the Kāpiti Coast, north of Wellington, has already been fixed at a cost of $2.3m, New Zealand Transport Agency project manager Chris Hunt said.

STUFF A screenshot from a 2017 video showing damage to the expressway.

"The Transport Agency takes a lot of pride in our work and we always aim to deliver great results for road users, so of course it's very disappointing when a problem like this develops."

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The expressway's slow lanes started leaking just months after opening in February 2017, resulting in discoloured cracks along the north and southbound lanes. Water leaking through a seal between the base – or pavement – and the asphalt was to blame.

MAARTEN HOLL/FAIRFAX NZ Take a time-lapse trip up the newly open Kapiti Expressway.

Hunt said the full repair cost was not yet known, but it would be shared by the agency and its contractors Fletcher Construction, Higgins and Beca.

"People expect better and that's why we have put so much effort into getting to the root of the problem, addressing it, and learning from it."

The resealing of the north and south-bound slow lanes, as well as the fast lane remedial repairs, were planned for completion by mid 2020.

VIRGINIA FALLON/STUFF The total cost of repairs is not yet known, NZTA said.

"The exact length of lane [kilometres] which may require remedial work will be determined after we have results from our monitoring of the pavement's performance over the winter months," Hunt said.

The information on repairs was released under the Official Information Act to former regional councillor Chris Turver who said it was an embarrassment the project had failed so badly.

"It's still a great new road for the country but the sheer scale of sealing failure raises many questions, including who was responsible and who will pay."

VIRGINIA FALLON/STUFF The discolouration is caused by water leaking through a seal between the base – or pavement – and the asphalt.

The agency said both it and its contractors were responsible for the "delivery of the project and the remedial treatment of all defects".

In October, Hunt said 14km of slow lanes would need resealing and contractors had been called in after the expressway showed signs of "minor fatigue" in May.

"This has subsequently loosened small particles in the top of the pavement, which causes the discolouration and texture changes," he said.

MAARTEN HOLL/STUFF The $630 million Kāpiti expressway opened to traffic in February 2017.

"The issue appears to have been caused by the unexpected penetration of moisture through the membrane seal."

The areas would be treated by reducing the number of joints in the pavement and joining sections together to make a more watertight and smoother surface.

Kāpiti Coast Mayor K Gurunathan said it wasn't the quality of the road, but the constant repairs that were causing frustration in the community.

"I always factor in an extra 15 minutes of travel time because of them."

Last month, those roadworks included contractors working on the Waikanae bridge joints in response to noise issues caused by the expressway.

Three other bridges were also recommended for treatment at a total cost of $1.4m.

The expressway will eventually link to the $850m Transmission Gully motorway to the south and a $330m northern section of the Kāpiti expressway running from Peka Peka to north of Ōtaki.