A police spokeswoman said they were notified at 11.10am on Saturday of a car in the water, near Weir Rd at Harington Point.
The car appeared to be empty.
"Police have assumed it was a stolen vehicle and likely dumped overnight," the spokeswoman said.
A tow truck had been requested to remove the car from the harbour.
An ODT photographer at the scene said there were skid marks that stretched about 10m down the road.
Empty beer cans were floating near the car and the air bags appeared to have been deployed.
A Fire and Emergency New Zealand (Fenz) spokeswoman said they were notified of the vehicle at about 9.45am and dispatched crews from Portobello and Lookout Point.
After locating the submerged vehicle, crews estimated it had been in the water for a number of hours and said it had been "likely dumped there", the spokeswoman said.
According to initial information, there had been four occupants inside the vehicle before Fenz had responded, the spokeswoman said.
No-one was inside the vehicle once Fenz arrived, putting to rest concerns it had been an accident, and the incident was left in the hands of police.
tim.scott@odt.co.nz , PIJF cadet reporter
