Two occupants, a male and a female, were inside the vehicle and lifeguards from the Sumner surf life saving club, police officers and members of the public got into the water to help the trapped pair, the witness said.
"It was pretty intense."
Rescuers were able to get the driver out, but tragically the woman was trapped in the submerged vehicle.
The witness told the Herald the section of the beach where the car entered the water had a notorious undercurrent and other people at the scene said they saw the car being swept out to sea before it went out of sight.
Inquiries will now be made to ensure further family members are notified.
Police extend their heartfelt sympathies to the family as they deal with this tragedy.
Canterbury District Police will continue to investigate the incident.