Hail and heavy rain have pelted parts of Sydney as storms sweep across NSW.

A reported tornado has swept through Dubbo and damaged several homes, while hailstones the size of 20 cent coins have pummelled parts of Sydney.

At least four people have been rescued from floods, including a woman pulled from a vehicle in Rouse Hill in Sydney’s northwest, SES spokesman Phil Campbell says.

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“These storms are bringing hail and reports of some flash flooding down near Wagga Wagga,” he said.

One woman posted a Twitter photo of hailstones beside a 20 cent coin as other pictures of hail blanketing backyards emerged on Monday night.

Sydney and the Illawarra region have been worst hit, but more than 100 calls to the SES have been scattered across the state, Mr Campbell says.

The Bureau of Meteorology has issued a flood watch warning for the region, along with the South Coast, Lachlan and Nepean River Valleys.

Rainfall totals in the range of 100-200mm are expected on Monday night and Tuesday, with higher localised falls of over 300mm possible, according to the BOM.

“Heavy rain and the potential for flash flooding (are) the main threats,” BOM regional director Stephen Lellyett said.