Work repairing two passing lanes on State Highway 1 near Waihola should be completed by Christmas, a New Zealand Transport Agency project manager says.
NZTA regional project manager Simon Underwood said seal on passing lanes at Clarendon (between Waihola and Milton) and another lane near Waihola (just south of the Henley turnoff) had been damaged by water.
About $50,000 would be spent repairing each of the lanes.
The 1km lanes were both completed early last year.
Road surface deterioration was "not a design or construction issue" and was caused by water from an unknown source.
"It is an unusual occurrence because damage to the surface has been happening in unusual places," Mr Underwood said.
"At Clarendon, it's the ridge of hill where we would not expect to find water. The two new passing lanes were completed last summer and have had ongoing issues with drainage.
"We are not sure of how water is making its way to the pavement surface. It was compacted to standard. It may have risen from a spring."
The pavement's upper seal layers were being repaired and stabilised, and drainage pipes installed.
The passing lane at Waihola cost $530,000. More extensive work at the Clarendon lane, which involved partial road realignment and installation of a passing lane, cost $1.9 million.
