A 100km oil slick through the North Island has been traced to a leaking container on a truck.
An estimated 2300 litres of transmission fluid was leaked from a truck heading southbound between Waiouru and Sanson, from 2.30am onwards.
Police say rain combined with the fluid to make the road extremely slippery.
Three or four cars had minor crashes and a truck jack-knifed on the road at Ohingaiti, south of Taihape, blocking the road completely.
Transport Agency regional manager Errol Christiansen says contractors are driving the length of the slick to find out how dangerous it is and how much work will be needed to clean it up.
SH1 is open to traffic in both directions, but extreme care is advised.