Four people are being taken to hospital after their car flipped in Oakleigh, near Whangarei, just before 8am.
At least one of the victims was trapped in the vehicle, a police spokeswoman said.
Fire, police and ambulance crew are all at the scene of the accident on State Highway 1, about 2km north of the Oakleigh overbridge.
Two people have serious injuries and two have moderate injuries, while a dog in the car has a broken leg, she said.
The SPCA has been notified of the animal's injuries. A tow truck is heading for the scene.
A police spokeswoman said it was not clear if the road had been closed but delays were likely.