Driving south in the slow lane of the Kāpiti expressway, in mid-September, showing some of the dusty spots of asphalt that are breaking up due to water penetrating the seal. The journey filmed runs between the Waikanae interchange and Raumati Straights.

The slow lanes on the freshly minted Kāpiti expressway are leaking, and 14 kilometres of it will need resealing, the NZ Transport Agency has revealed.

Project delivery senior manager Chris Hunt said a fix would require resealing long stretches of the 18km expressway in both directions, and the agency would be footing its share of the bill.

The expressway, through the Kāpiti Coast north of Wellington, started developing discoloured cracks only months after it opened in February.

STUFF A section of the deteriorating slow lane on the Kāpiti expressway, at Paraparaumu.

In June the agency confirmed work would be needed to address "pavement issues" along about 1.26km around the Kāpiti Rd interchange in Paraparaumu.

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On Friday, Hunt said the repair method would now "require around 14km of both the north and southbound slow lanes to be resealed".​

CAMERON BURNELL/STUFF Kāpiti Coast Mayor K Gurunathan says the latest problem with the expressway is "unfortunate", and he expects the new seal will be of the same low-noise grade as the first.

The members of the expressway Alliance, a consortium of contractors and NZTA, would be sharing the costs of the work.

Hunt said contractors were called in after the expressway showed "signs of minor fatigue" in May. Water was leaking through a seal between the base – or pavement – and the asphalt.

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

JOEL MAXWELL/STUFF Before: The expressway near the Kāpiti Rd interchange at Paraparaumu immediately after its opening.

Hunt said there were 3.2km of affected seal, spread over the expressway's slow lanes, which experienced the heaviest traffic.

"These areas will be treated by reducing the number of joints in the pavement, and joining sections together to provide a more watertight and smoother surface for motorists."

This would amount to about 14km of slow lanes being resealed. The repairs were expected to be completed by the end of the summer work season, next year.

STUFF After: The slow lane near the Kāpiti Rd interchange in mid-September, showing tell-tale signs of deterioration.

"Until we're able to define the exact extent of the issue and the areas for remedial works, the cost is unknown."

The expressway, one of the Government's roads of national significance, was completed months ahead of schedule, but Hunt said the application of the seal was not rushed.

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

MARK COOTE During construction, the Poplar Ave interchange for the Kāpiti expressway, looking west. Poplar Ave is one of the main trouble spots for the leaky seal.

Kāpiti resident Ian McIntosh, who commutes to Wellington four days a week, said the road had been "breaking up ever since it opened".

"They are cracks in the surface, rather than big potholes. It's as if the road is waterlogged underneath.

"A lot of the discolouration, you'll find it on the shoulder. It's as if, when traffic goes across the deteriorating surface, it's almost squeezing water out."

McIntosh said that, apart from a few initial congestion problems, the expressway had made a big difference to his commute, eliminating congestion between Paraparaumu and Waikanae.

​Kāpiti Coast Mayor K Gurunathan said the problem was "unfortunate", and he expected the new seal would be of the same low-noise grade as the first.

Politicians had a habit of putting pressure on NZTA, he said, but he did not know whether that was behind the early completion of the expressway.

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

On Thursday the agency launched nationwide consultation on increasing the speed limit to 110kmh on two sections of suitable roads, including part of the Waikato expressway.

The Kāpiti expressway would be considered for a 110kmh speed limit in the future, it said.

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