A female firefighter in Australia has been hit by a burning branch that fell from eight metres on top of her as she helped fight an out-of-control bushfire that is threatening homes in Sydney's northwest.

An emergency warning is currently in place for the fire in Londonderry as hundreds of firefighters battle the blaze which is burning close to homes.

Resident Matt Refalo heard the screams from the firefighter and ran down to see if he could help on Government Road Llandilo just after 4.10pm (Aust time).

The Daily Telegraph

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witnessed firefighters pull a burning branch off the woman who was helping mop up.

She is currently being treated by RFS at the scene and an ambulance is en route.

The top of the tree burnt through causing the branch to drop.

Residents of Fourth Ave, Spence Rd, Llandilo Rd and Mayo Rd in Londonderry are being urged to shelter as the fire approaches as it is too late to evacuate.

Fire crews have brought in water bombers to tackle the out-of-control bushfire, which is ripping through Castlereagh Nature Reserve, trapping residents in their homes.

The Rural Fire Services has sent out alerts to families to shelter because it is too late to evacuate.

"It doesn't get much more intense than this," a RFS spokesman said.

"It's happened so quickly that people have not had time to evacuate."

As well as water bombing helicopters, the Hercules dubbed Thor and the Southern Belle DC10 are in the air dumping water.

Firefighters believe the fire began along The Northern Road and quickly spread to the nature reserve, putting homes at risk as far as Stoney Creek Road to the east, Ninth Ave to the south, The Northern R to the west and around the reserve.

The fire is being fanned by 30-35km/h western winds.

Temperatures in the area are approaching 30 degrees.

It comes after a number of major fires raged across the state last week and gave a glimpse of what could come this summer.

Fires near Port Stephens and Cessnock and on the South Coast caused evacuations, damaged property and burnt out hectares of bushland.