Water bombing helicopters have been called in to assist firefighters in a bushfire in Sydney's north.

The blaze is burning out of control on the Bluff track at Forestville.

The Rural Fire Service (RFS) says the blaze is heading towards Grattan Crescent and Moira Place.

No homes are under threat at this stage but a 'Watch and Act' alert is in place.

Firefighters have been battling the blaze this afternoon and helicopters have started dumping water on the fire.

A spokesperson says temperatures are starting to drop, and the cool change is assisting fire-fighters in their efforts to control the blaze.

It is not clear how the fire started.