The mop-up resumed at first light after severe thunderstorms battered the state overnight.

Power was cut to about 11,000 homes and businesses in western Sydney and the Illawarra on Tuesday evening, and emergency crews from Endeavour Energy worked to restore power to all but 2,100 customers just before 11pm.

Several hundred customers across the storm-affected area would not have their power restored until Wednesday morning, Endeavour Energy said.

The Bureau of Meteorology had issued a severe thunderstorm warning just after 6pm on Tuesday for large hailstones, damaging winds, heavy rainfall and possible flash flooding.