The roof of a hospital has partially collapsed in the Blue Mountains, after wild weather lashed Sydney's western fringe.
The weather bureau says a severe thunderstorm is producing large hailstones and heavy rain that could lead to flash flooding in parts of the metropolitan, central tablelands and Hunter forecast districts.
Fire and Rescue superintendent Paul Johnston says emergency services are working with engineers at the hospital in Katoomba.
"To help clean up the area, remove water, stop more water getting in," he said.
"In the hospital there's been no evacuations... but 10 patients have been relocated within the hospital."
The State Emergency Service says it is receiving a high volume of calls and is asking residents to be patient, or to call triple-0 in an emergency.
© ABC 2014
12:37 EST A deep low pressure trough created a light show for residents and visitors alike about northern parts of New South Wales, with some areas picking up daily rain not seen since April.