Motorists taking State Highway 29 over the Kaimai Range have been warned of an oil spill from an accident that has already caused several other crashes.
Police said the spill had spread by up to 300 metres and the road was down to one lane while contractors cleared it up.
The spill happened after a crash between a car and a ute, just after 10am.
Police said there had been three other minor crashes since the initial accident, but no injuries have been reported.