Auckland's northern motorway has been reduced to one lane after a small truck flipped in the south bound lane at Dairy Flat.

The truck's single occupant was uninjured and emergency services were attempting to clear the lanes, police said.

A diversion for southbound traffic was in place which takes motorists off at Silverdale and onto Hibiscus Coast, onto East Coast Road, and back onto the Northern Motorway at Oteha Valley Road.

Due to the congestion, police advised motorists to avoid the area.