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'."
"The car was rocking, stopping, rocking, stopping."
The man said he told the couple in the car he would drive his van closer and secure a rope to their car to stop them going over the edge.
But before he could get his van into place, the car was already in "the drink".
The car then floated off with the wind taking it out to sea towards moored yachts.
The car can be seen in the harbour. Photo / Michael Craig
The couple scrambled out their windows and on to the roof before the man called to them to swim to shore before it sank.
"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.
He first noticed something was up when he saw a black object floating out of the corner of his eye.
Then about a minute later, water flowed through the open windows and the car submerged quickly.
The SUV had by that time floated about 60m offshore and maybe 120m from where it slipped into the water, Cross said.
The back end of the car could earlier be seen just poking above the harbour waters.
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.
