Heavy but brief rain that moved through Sydney has brought down part of a hospital roof and forced affected patients to be moved.

The storm from the west moved through the Sydney basin on Tuesday afternoon, bringing hail to some areas and downpours before it moved off the coast.

The roof over a ward in Mt Wilga Private Hospital, a rehabilitation centre in Hornsby in Sydney's north, collapsed around 6pm (AEDT).

A hospital spokeswoman said no one was injured but two patents had to be relocated within the hospital.

The NSW State Emergency Service (SES) responded to a flash flooding incident in Flemington in western Sydney. They rescued a person from a car.

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An SES spokesman said little damage was reported since the rains were brief and without strong winds.

AAP