Kerikeri, Kawakawa and Kaikohe fire brigades were called out.
Kerikeri fire chief Les Wasson said firefighters extricated the woman from the vehicle and onto a stretcher.
The car, thought to be a Honda Civic, was well off the road and not blocking traffic.
A St John spokeswoman said two ambulances and a helicopter responded.
One patient in a critical condition was flown to Auckland Hospital by the Northland Rescue Helicopter.
The police investigation is continuing. A lack of skid marks suggest the crash may have caused by a medical event but that has yet to be confirmed.