Fierce storms have hit Sydney's south-west this afternoon, causing flash flooding and injuries from lightning strikes.

A man has been taken to Campbelltown Hospital in a stable condition after being struck by lightning in Campbelltown.

Sydney's south-west was hardest hit by the storms.

The ambulance service says paramedics are also assessing 11 children who suffered minor symptoms after the room they were in at Heathcote was struck by lightning.

At Broughton Anglican College in Campbelltown several cars were submerged in the school's carpark.

About 8,500 homes have lost power in the area, and Cityrail has warned commuters to expect extensive delays.