A truck has rolled on State Highway 26 south of Paeroa, blocking part of the road.
A police spokesman said a car involved in the incident at 7.15am was in a ditch.
Two people have been taken to Waikato Hospital, both in a serious condition.
Update: #SH26 near Tirohia, south of Paeroa has stop/go traffic management in place following a truck crash. Avoid or expect delays ^TP— NZTA Waikato/BoP (@NZTAWaiBoP) April 3, 2016
The spokesman said one lane of the road, near Tirohia, was blocked.
Motorists should expect delays.
At 7am, NZTA reported a car has collided with a transporter on SH 73 in Kirwee, near Christchurch.
A St John spokeswoman said one person had been taken to hospital with minor injuries.