NSW State Emergency Service (SES) is warning afternoon storms will continue over the next several hours.
Severe thunderstorms are likely to produce damaging winds, large hailstones and heavy rainfall that may lead to flash flooding in the Southern Tablelands and parts of the Illawarra, South Coast Sydney Metro area, Central Tablelands, South West Slopes, Central West Slopes and Plains and Snowy Mountains districts.
Across the state, volunteers have responded to 85 incidents in the last 24 hours. These have mostly been for fallen trees and branches and leaking roofs.
NSW SES State Duty Commander, Chief Superintendent Dallas Burnes, said this is a dynamic weather event and conditions will change quickly.
"We're asking people to monitor conditions and be flexible as the situation changes," he said.
"We may see some heavier downpours of rain which could see water over roads and flash flooding with very little or no warning.
"If that occurs, please do not drive through flood waters. Stop, turn around and find an alternative route. If trapped on a flooded road call 000 immediately."
The NSW SES is advising people to avoid unnecessary travel, move indoors and keep children and pets inside.
Park your car under secure cover and away from trees, powerlines and drains.
If safe to do so, secure or put away items, such as outdoor furniture, trampolines and roofing iron that could blow around in strong winds.
For the latest information and warnings visit ses.nsw.gov.au.
For help in floods, storm and tsunami call the NSW SES on 132 500 or in life threatening situations call Triple Zero (000).
