A Sydney woman is in hospital after being impaled by a falling tree branch and pinned against a car.

The 55-year-old was standing between two parked cars on Gouldsbury street in Mosman on Saturday when a large branch pierced her through the stomach and trapped her against one of the cars.

Paramedics freed the woman and took her to Royal North Shore Hospital, where she is in a serious but stable condition.

Firefighters had to secure the tree after it dropped several other branches.