Two trucks have collided on Greta Road, Greta Valley, in Hurunui around 9:10am.
One truck has rolled and the driver is believed to be in a critical condition.
"Fire and Emergency NZ and ambulance are also at the scene," says a police spokesperson.
"Diversions are being put in place and the road may be closed for some time.
"The Serious Crash Unit are attending the scene."