Highways are clogging up north of Wellington as motorists head off for the city's anniversary weekend.

Traffic "was starting to get heavy" north of the capital and a crash on State Highway 1 near Ohau was causing further delays this afternoon, a New Zealand Transport Agency spokesman said.

The crash happened north of Otaki.

Police Central Communications said the crash involved a vehicle colliding with a pole.

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One man was thought to have leg injuries after the crash but police said the accident scene would be cleared shortly.

Wellington Anniversary Day is this Monday.