The New South Wales State Emergency Service (SES) is warning people in Sydney, the Blue Mountains and the Hawkesbury, to prepare for severe thunderstorms this afternoon.
The weather bureau has issued a storm warning for large hailstones, heavy rainfall, flash flooding and damaging winds.
The storms are moving north-east of Sydney, towards Windsor and the Yengo National Park.
The SES is advising people to move their cars under cover and keep clear of flooding creeks.
A severe storm in the state's central-west has already left 5,000 homes without power.
There was flash flooding and trees were brought down when the storm hit Bathurst just before 2pm (AEDT).
The SES has received nine calls for help but says it expects that number to rise.
Essential Energy says several crews are working to restore electricity after trees fell on powerlines in the city.
© ABC 2013