A large offal spill on the road between Kaiwaka and Wellsford has closed State Highway 1, causing significant delays.

The spill stretches about 4km and is expected to take several hours to clear.

Police staff are in attendance and council contractors have been advised.

Diversions are in place off SH1 southbound at Kaiwaka-Mangawhai Road, and northbound at Mangawhai Road.