Heavy rain on the NSW south coast will ease by nightfall, with the torrential conditions sparking a warning to stay indoors and avoid unnecessary travel.

A severe weather warning is current for the Illawarra region south of Sydney due to the risk of flash flooding.

Six-hourly rainfall totals up to 120mm are possible, the Bureau of Meteorology said in an alert.

It warned the hardest-hit areas are likely to be Wollongong, Bulli, Port Kembla, Albion Park, Kiama and Huskisson.

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The overnight deluge set a new daily rainfall record at Point Perpendicular at the northern tip of Jervis Bay which received 143mm of rain in the 24-hours to 9am on Sunday, meteorologist Edward Medlock said.

Conditions are predicted to ease in coastal areas from Sunday afternoon.

Medlock explained the brief reprieve was due to a trough moving north to Sydney, but warned heavy rain would return to the coast later in the week.

“There’s no let up for the showers for the current forecast period but the large totals are expected to ease tomorrow,” he said.

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The NSW State Emergency Service told residents and holiday makers in the Illawarra to stay indoors and avoid unnecessary travel.

“Park your car under secure cover and away from trees, powerlines and drains,” the service said in an alert.