Heavy rain, lightning and winds as strong as 100 kilometres per hour (60 miles per hour) have swept across Sydney and other areas of New South Wales since Wednesday.

With many power lines felled, about 30,000 properties remained without electricity Saturday -- down from a peak of more than 260,000, said the state's emergency services minister, Jihad Dib.

"This is an incident that is affecting the whole state," he told reporters.

Emergency services had responded to more than 7,000 incidents around New South Wales, he said.

"We know that it has not been an easy thing to go through."

Disasters had been declared so far in three local government areas, he said, unlocking support for people seeking emergency housing, essential items, repairs and clean-ups.

Electricity network Ausgrid said 140,000 customers had lost power Wednesday and another 68,000 since Friday, with some areas experiencing winds of up to 100 kph.

Police reported that an elderly man was killed in the storms when his car was hit by a tree on Wednesday in Cowra, about 230 kilometres (143 miles) west of Sydney.