A huge huntsman spider caused a chain reaction which finished with a teenage Volkswagen driver watching helplessly as her car rolled into the river.

The 18-year-old woman had driven her vehicle to the Bayview Boat Ramp, on the northern beaches, in the early morning when the arachnid fell from underneath the visor and onto her lap.

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The panicked woman applied the handbrake only half way before leaping from the car and doing a "spider dance" in an attempt to rid herself of the spider, witnesses said.

But while she was occupied with the creepy crawly her car began slowly rolling into the water.

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The car quickly rolled into the water and drifted away. Picture: 7 News

Witness Stuart Cooper said he feared the worst when he saw the submerged vehicle.

"I sort of ran over towards yelling out 'was there anyone in the car' and she goes 'no no no a spider dropped on me'," he told 7 News.

"She was really upset and shaking and crying."

The vehicle quickly sunk and needed to be towed back in by police.

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The spider is believed to have been a large huntsman.