An out-of-control grassfire is burning at the Barrenjoey Headland, near Palm Beach, on Sydney's northern beaches.

The NSW Rural Fire Service has issued an emergency warning for the fire, which began around 2:00pm (AEST).

Firefighters are on the scene and are being assisted by two helicopters.

Authorities are warning anybody in the area to take shelter at the Barrenjoey lighthouse.

They say not to try to walk back through the bush because the fire has cut access to the walking track.

More to come.