The NZ Transport Agency says a truck and trailer has rolled on State Highway 1 in the Dome Valley between Warkworth and Wellsford, completely closing the highway for south and north bound traffic.
The crash happened about 2.45pm near Falls Bridge, and the NZTA says the highway could be closed for four hours.
All traffic is being diverted on to State Highway 16. People heading north are being diverted along Kaipara Flats Road, just north of Warkworth, to SH16. Those driving south are being diverted from Davis Road in Wellsford.
The NZTA says SH16 is a rural highway and people should drive with care because of higher volumes of traffic using it.