Motorists travelling on State Highway One through Horowhenua are facing a slow tip because of surface flooding this afternoon.

Police said, about 4pm, both lanes of the road were covered in water over about 1km near Manakau, south of Levin.

"Traffic is backed up and the southbound lane is the most affected," a police statement says.

"Roading contractors are on the way to help begin clear the surface flooding."

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Police are advising motorists to delay travel or avoid the area if possible. Those driving through can expect delays.