A teenage arachnophobe has watched her car roll into a NSW river after jumping out to brush a large spider off her body.

The 18-year-old had just pulled up to the Bayview Boat Ramp in Sydney's northern beaches at 7am when a huntsman spider crawled out from under her visor and jumped onto her lap.

With only enough time to apply the handbrake half way, the terrified teen jumped out of the car and frantically tried to brush off the large arachnid.

As she danced around on the boat ramp, her Volkswagen began to roll down the ramp and into the cold water.

By the time she realised what had happened the car was out of reach and afloat.

"It was like it all happened in slow motion," one witness told News Corp yesterday.

When police divers arrived at the scene the car was completely submerged and needed to be towed ashore.