Traffic on a busy section of the highway north of Wellington has slowed to a crawl after a car crash.

Transport authorities are warning motorists to expect "significant delays" on State Highway One, after the crash on the bridge south of Waikanae on the Kapiti Coast about 4.30pm today.

The road was opened to traffic in both directions about 5pm, but the NZ Transport Agency was still warning motorists to expect a slow trip through Waikanae.

Photo / Supplied
Photo / Supplied

By 5.15pm, a trip from the Wellington CBD to Waikanae was taking more than 90 minutes.

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A witness has reported that a car heading north crossed into the southbound section and collided with another vehicle.

Waikanae man Kevin Aanensen saw the aftermath of the crash.

"There's a vehicle coming from the south. Looks like it's gone across into the southbound lane. It's just on the end of the bridge, right outside our house on the north side of the bridge."

All emergency services were at the scene.

The car that was struck was driven by a female driver, who appeared to have chest injuries, Mr Aanensen said.