Police and the owners have already left the scene, but a tow truck is now at the site pulling the car back towards the shore with a rope that had been attached by an diver.
The car park at the reserve has no railing preventing cars from driving off the edge.
A witness who did not wish to be named said he was asleep in his house truck when he heard a scraping sound.
Northcote Point.
"I looked out my window and I could see the car teetering on the edge of the sea wall here," he said.
"There were two people in the front and I yelled, 'don't move'."
"So they ran across the roof, jumped in the water and swam to shore where they climbed up the sea wall."
In an earlier statement, police said: "Police received a report of a vehicle that had gone into the water near Northcote Point at about 1.45pm.
"All occupants of the vehicle are on the shoreline. Police are at the scene. There are no reported injuries."
The witness said the car kept floating before eventually starting to sink.
The submerged car at Sulphur Beach, Auckland. Photo / Michael Craig
The couple were in a bit of a flustered state by the time they came to shore but said "something about a mat" getting stuck and stopping them pushing the brake pedal down, he said.
But an hour later as the tide edged out, its passenger windows and roof could also be seen above the waterline as its wheels came to a rest on the harbour floor.
Cross also said it was very lucky the couple had wound their windows down before the car went into the water.
The tow truck company said it planned to pull the car closer to the shore before loading it onto a truck using the boat ramp.