The central regions of NSW are also forecast to cop a downpour on Thursday.
The Bureau of Meteorology says an approaching upper trough is combining with an unstable environment to trigger severe thunderstorms across NSW.
"A severe thunderstorm warning continues for very dangerous slow-moving storms between Wollongong and Sydney," the BOM said on Thursday.
"Intense rainfall and flash flooding are likely."
Weeks of sunshine throughout NSW were cut short on Thursday with Bellambi recording 95 mm of rainfall in one hour, 73 mm at Shellharbour Airport in an hour and Cringila hitting 90 mm in two hours.
Sydney, Parramatta, Wollongong, Campbelltown, Kiama and Huskisson are predicted to be hit with intense rainfall on Thursday afternoon.
Floods have cut off roads from Coalcliff to Clifton, in Berkeley and Dapto as well as from Thirroul to Stanwell Park.
The SES says its Wollongong, Dapto and Shellharbour Units rescue units have been dispatched to 235 incidents.
Edmund Rice College in Wollongong was evacuated after flooding and Towradgi Public School contacted parents about midday asking that children be collected early because of the storm.
On social media, residents posted images of floating cars caught in floodwaters metres away from a beach while others posted landslides of cracked roads.
A heavy rainfall warning has been issued for areas of the Central and Southern Tablelands including Orange, Bathurst, Yass, Blayney and Trunkey Creek.
In November, about 20,000 residents in central west NSW, particularly in Bathurst, were left without hot water to shower or ovens to cook with for weeks because of floods hitting a crucial gas pipeline.
The SES warns flash flooding could extend to Nowra, Port Kembla, Albion Park, Kiama and Huskisson.
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