The alarm was raised at 6.45am on Wednesday by people who saw the car go into the water off Westhaven Drive.
One bystander jumped in to rescue the driver, who was taken to Auckland Hospital in a serious condition.
The passenger, who freed herself, has been taken to hospital for a checkup.
Inspector Willie Taylor says it looks like a pure accident.
He says the pair are lucky to be alive.
Mr Taylor says the area is busy at that time of the morning with runners, cyclists and commuters but if the accident had happened an hour earlier there would have been noone around.
He says the car is completely submerged and is being secured by a dive team.