Emergency services are responding to a serious crash involving a truck and a car on State Highway 1 (Omihi Road), Greta Valley.
Two people are reported to be in a critical condition and a helicopter has been deployed.
A further two people have minor injuries.
The road is blocked, and diversions will be in place for several hours.
The diversions are at Waipara and Greta Valley on State Highway 1.
Motorists are advised to expect delays and to avoid the area.