A NSW south coast public school has been closed as a nearby dam wall threatens to break, in the midst of severe weather pummelling the region.

The SES issued an urgent evacuation notice to residents living downstream from Jerrara Dam, west of Kiama.

The dam wall has shown signs of crumbling as heavy rain continues to fall in the area.
The nearby village of Minnamurra would bear the brunt of it if the dam wall collapses, with the SES advising parents with children at Minnamurra Public School to collect them.

A spokeswoman at Minnamurra Public School, which has 430 students, said it's a precautionary measure.

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If the dam busts, it will cause extremely dangerous flooding to areas including Mt Brandon Road, Jerrara Road and Jamberoo Road, an SES spokesman said.

"We do not know what is going to happen," he said.

"The dam is currently being decommissioned and we are noticing the water is spilling over the top and is too risky, we are asking residents to evacuate immediately."

Lisa, who works at Jamberoo Bowling Club, said if the dam wall goes there is a concern that water will flood the Minnamurra River which will flood low-lying parts of Kiama Downs.

She said parents have been told "Please don't panic, but please pick up children from school".

"The biggest issue we have is for parents that work out of town and left this morning and can't get back in ... like what happened a couple of years ago," she said.

Overnight SES responded to more than 600 call-outs and helped with 34 flood rescues, including 28 in Sydney's west and east, as heavy rain and hail lashed parts of NSW.

Up to 50 homes have been left isolated by flood damage in Kiama.

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Residents south of Wollongong and Kiama were bracing for more heavy rain.

Parts of the Illawarra have had falls of up to 200mm to the 24 hours to 9am.
The BOM issued a flood watch warning for the region, along with the south coast and Illawarra.

SES Deputy Commissioner Greg Newton said all properties that could be affected by flooding from Jerrara Dam have been evacuated.

"Controls have been placed in that area to make sure people don't back into that area or travel on any of the roads that might be at risk should we get significant flooding if the dam wall fails completely," he told ABC radio.