Two light planes were understood to have collided and crashed before midday in Sydney's outer southwest.

Members of the public were urged to avoid the area and no further information was available at this stage, NSW Police said.

NSW Police, Fire and Rescue and NSW Ambulance were attending the scene, with at least two firetrucks and several ambulances present, Xinhua news agency reported.

An investigation into the cause of the crash would be undertaken by the Australian Transport Safety Bureau, NSW Police said.