The incident happened near the small town of Ruatapu, in the Westland District, yesterday afternoon.
Emergency services who attended thought she may have been checking if a door was secure when she fell out, but the circumstances were unclear.
The woman was travelling with her male companion, a 10-month-old baby and mother-in-law, and is thought to be European.
Tourists came upon the scene immediately afterwards and called for help at 3.56pm.
They said the woman was lying on the roadside with head injuries and was bleeding heavily.
"Her pants were shredded and she had cut knees," a witness told the Greymouth Star.
The campervan was parked about 50 metres further along the road and her partner was holding her.
The tourists said the whole scene was "a bit of a shock".
St John ambulance, police, Fire and Emergency NZ and the Roa Mining West Coast Rescue Helicopter attended.
Volunteer fire crews from Hokitika provided a landing zone for the helicopter and secured the scene, performing traffic control.
State Highway 6 was closed briefly while the chopper landed and the woman was flown to Te Nikau Hospital in Greymouth.
