More than 7100 insurance claims were lodged on Tuesday as rescue workers battled wind and torrential rain to respond to more than 5500 calls for help around the state.
NSW State Emergency Service deputy commissioner Steve Pearce estimated the bad weather had caused millions of dollars in damage to property and infrastructure.
"Damage costs for this event are already in the millions and are expected to rise as the storms continue to strike this evening," Mr Pearce said.
The Insurance Council of Australia said insurance companies had received about 7100 claims on Tuesday for storm-related damage across NSW including the Sydney metropolitan area, the Hunter and Illawarra.
"Most of the damage is for fairly light damage to homes, mainly water damage and other damage caused by wind," an insurance council spokesman said. "There has also been damage to motor vehicles."
Networks NSW estimated that the overall repair bill to electricity networks for Ausgrid and Endeavour Energy would be many millions of dollars. About 8000 Endeavour Energy customers were without power throughout the day.
A spokeswoman for Networks NSW said severe weather was continuing to cause major damage to the electricity network across Sydney, the Central Coast and Hunter region, leaving about 200,000 homes and businesses without power.
"Customers should prepare to be without power throughout the night while crews work through more than 4500 reports of urgent hazards, including trees bringing down live powerlines," she said.
"Storm activity is forecast to continue this evening and into tomorrow, causing further damage to the electricity network, and it's likely to be several days before electricity supply is restored to all homes and businesses.
"Ausgrid thanks customers for their patience and support for crews as we prioritise these emergency repairs to help keep the community and our staff safe."
Customers were urged to only phone through to operators to report hazards and emergencies on 13 13 88.
Power workers have been battling to restore electricity to thousands of homes and businesses after the weather felled trees and power lines.
The Electrical Trades Union said thousands of its members responded to hundreds of calls for assistance after heavy rain and strong winds cut power supplies.
"While most people were in bed last night, thousands of electricity workers worked through the night in dangerous conditions to protect the public from downed power lines and to restore power to more than 130,000 homes and businesses," ETU organiser Mark Buttigieg said.
"What we saw last night was nothing short of devastation, and the clean-up task - including the restoration of power - will continue for several days to come.
"As always, electricity workers put the community first by working around the clock responding to emergency call outs, and the ETU believes these workers deserve recognition for their selflessness and committed service to the public in extremely dangerous conditions."
Brian Seidler, executive director of the Master Builders Association, said storms forced delays in major structural work on building sites around the state.
"This sort of weather would be impacting on most building projects, particularly the ones coming out of the ground," he said.
"All excavation work under these terrible conditions would be difficult to continue. Any exposed building processes, brick work and standing of scaffolds would be hazardous to undertake."
