A Hato Hone St John spokesman said they responded to the incident with one ambulance and one helicopter. The woman was airlifted to Hawke's Bay Hospital in a serious condition.
A statement from Trust Tairawhiti Eastland Helicopter Rescue said a second helicopter from their sister base in Taupō came to assist due to the critical nature of the patient's condition and flew the patient to hospital.
The statement thanked two doctors from Wairoa and Hawkes Bay for their help on the way into work, as well as St John and FENZ.
A Fire and Emergency New Zealand spokesperson said firefighters were also called - one fire truck attended the scene and left at 8.16am.
Wairoa mayor Craig Little said he drove by the scene of the incident about 8am and saw a vehicle "well below the road".
"I went past the Mohaka Hill, probably 15 minutes outside of Wairoa, and there was a car down the bank," Little said.
"It looked like a considerably good-sized car and you could see the undercarriage, so it was up on its side or top."
