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

Transport authorities are warning motorists to expect significant delays on State Highway One, after the crash on the bridge south of Waikanae.

Emergency services are at the scene.

The southbound lane is blocked and a stop/go system is in place to let traffic through.

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