"We are treating this incident as a hit and run.
"The driver has fled the scene before checking on this woman's wellbeing or calling for help."
The person who found her initially thought she was a pile of clothes on the road, Mr Symonds said.
Police had identified the woman but would not be releasing her name until the exact nature of her injuries had been established.
She was in a critical but stable condition in hospital.
Police appealed to anyone who may know anything about the incident to come forward.
"We believe the car involved in the incident will have damage to the exterior, such as dents to the bumper or bonnet, and possibly blood on it.
Anyone with information should contact Counties Manukau Police on (09) 261 1300 or Crimestoppers, anonymously, on 0800 555 111.