A severe weather warning for thunderstorms with the possibility of large hail stones, heavy rain and damaging winds has been issued for the Illawarra and other parts of NSW.
The Bureau of Meteorology warns severe thunderstorms on Friday night may lead to flash flooding over the next few hours in Newcastle, Gosford, Sydney, Wollongong, Nowra and Katoomba.
The NSW State Emergency Service is advising people to move their cars under cover or away from trees, secure or put away loose items around your house, yard and balcony and keep at least eight metres away from fallen power lines or objects that may be energised, such as fences.
Fallen power lines can be reported to either Ausgrid (131 388), Endeavour Energy (131 003), Essential Energy (132 080) or Evoenergy (131 093) as shown on your power bill.
The SES says trees that have been damaged by fire are likely to be more unstable and more likely to fall.
During and after storm events people should keep clear of creeks and storm drains and not walk, ride a bike or drive through flood water.
If you are trapped by flash flooding, seek refuge in the highest available place and ring 000 if you need rescue.
For emergency help in floods and storms, ring the SES (NSW and ACT) on 132 500.