UPDATE: Sydney sub-station fire under control after lightning strike

A fire at a sub-station in inner Sydney which was sparked by a lightning strike has been brought under control.

More than 30 Fire and Rescue NSW firefighters worked to contain the blaze in Pyrmont.

Power has been restored to homes in the area as crews reroute the electricity supply. At least 25 units were evacuated as crews worked to bring the fire under control.

The Bureau of Meteorology earlier issued a warning for parts of NSW for large hailstones, heavy rain and flooding.

Heavy rain has caused flooding in the main street of Mittagong, in the NSW's Southern Highlands.

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Fire and Rescue NSW responded to more than 80 storm-related incidents including trees and wires down, automatic fire alarms and flooding.

The SES responded to more than 350 calls, with the worst-hit areas in Blacktown and Parramatta.

So far, 13,000 homes have had electricity cut in the Blue Mountains, Hawkesbury, Southern Highlands and western Sydney regions.

More than 16,000 homes were without power at the height of the storms.

The storms have also caused some delays to flights out of Sydney Airport with Qantas and Virgin confirming that its domestic and international runways had closed since 5.15pm.

Qantas said about 10 flights had been affected, some by up to two hours.

By 7pm, a Virgin spokeswoman said its flights had returned to normal with the runways reopening.

Meanwhile, trains are experiencing delays between Richmond and Clarendon due to lightning strikes at Clarendon.

Buses are operating a replacement service between the stations, however heavy traffic in the Richmond area is causing delays for the buses.

There is also heavy traffic delays in Strathfield due to flooding.

Additional reporting by AAP