The Bureau of Meteorology advises Flash flooding is a risk over the next few hours with parts of Metropolitan Sydney, Hunter, Central Coast and Central Tablelands of particular concern.
NSW SES Assistant Commissioner Nicole Hogan said the heavy rain could inundate drainage systems and cause water over roads.
"As drainage systems become overwhelmed, we could see road closures without much warning," Assistant Commissioner Hogan said.
"If you don't need to be on the roads during heavy rainfall, we ask the community to delay their travel until the storm passes."
Assistant Commissioner Hogan said flood rescue crews have been prepositioned in the high-risk areas.
"While crews are ready to respond to an increase in calls for assistance, we urge the community to steer clear of floods" she said.
"If you do come across flash flooding, the best thing to do is turn around and find another way - It's not worth putting your life or your loved ones at risk.
"It doesn't take much for vehicles to become stuck in water or washed away as you don't know what damage has been done to the road surfaces.
Assistant Commissioner Hogan also advised holidaymakers planning to travel, to monitor conditions, not just for their destination but for the roads on the way.
"Be sure to check Live Traffic and council websites for any closures before your journey and follow any road closed signage you come across," she said.
"Download the Hazards Near Me App and set up a watch zone so you can stay across the latest information and warnings."
Conditions are expected to ease late Friday, before another bout of thunderstorms is predicted for many areas over the weekend, shifting to the north on Monday.
If you require assistance from the NSW SES call 132 500. If it's a life-threatening emergency, call Triple Zero (000).
