The Bureau of Meteorology cautions that Saturday could bring periods of intense rainfall leading to life-threatening flash flooding in the Illawarra.

A severe weather warning for heavy rainfall has been issued for the Illawarra, as well as the Mid North Coast, Hunter, Sydney and Central Tableland districts.

Already the Mid North Coast has been hit with significant falls, with some places - Utungun and Upper Rollands Plains - receiving more than 200 millimetres within seven hours.

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The focus of the heavy rain this evening is between Kempsey and Bulahdelah, but this will shift on Saturday to the area between Newcastle and Kiama, including Sydney and Wollongong.

In this area, the Bureau says, roads are likely to be cut off and there will be an increased risk of landslips.

The State Emergency Service advises that people should:

Earlier

With a torrential downpour expected on Saturday and heavy rain forecast either side, Illawarra roads were already beginning to be impacted on Friday.

As of Friday morning, flooding was already prevalent at Albion Park Rail with the RMS closing the Illawarra Highway between Croome Lane and Taylor Road.

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Between 100mm to 200mm was predicted to fall across Albion Park on Saturday and up to 120mm further south at Nowra, according to the Bureau of Meteorology. The State Emergency Service is warning Illawarra and South Coast residents to be prepared for flooding and limit travel - especially on Saturday.

A hazardous surf warning, marine wind warning and flood watch has been issued for the region for Saturday.

"A coastal trough has intensified about the northern coast overnight, bringing heavy rain to the area," the Bureau's website stated.

"Within this trough a low is forecast to develop, slowly tracking south along the coast on Friday and Saturday, with widespread heavy rain expected to spread south along the coast and adjacent ranges.

"Conditions may ease somewhat later on the weekend as the trough begins to weaken."

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A spokesman for the SES said it was the middle of storm season so water catchments and the earth were unable to absorb a lot of water, therefore causing risk for flash flooding anywhere at anytime - such as an overflowing storm water drain.

"Treat it as though it could happen anywhere," he said. "We're actively encouraging people to reassess the need to travel ... and if need do need to be on the roads please do not drive, ride or walk through flood water."

The SES has had 1300 requests for help since midnight on Thursday - mainly in the north - though they expect their Illawarra and South Coast volunteers to be busy across the weekend.

The Bureau is forcasting showers and chance of a thunderstorm for Friday, with heavy falls on Saturday along with southeasterly winds up to 45 km/h. The amount of rain is set to ease on Sunday, but showers are forecast throughout next week.

For emergency help in floods and storms, call the NSW State Emergency Service on 132 500.

In life threatening situations call Triple Zero (000) immediately.

For more information:

Flash flooding may also impact public transport, including trains. Please visit www.transportnsw.info for the latest information.

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