Illawarra residents can expect heavy downpours, with some wind.

On Thursday, there is a 95 per cent chance of rain, with up to 25mm possibly falling and a top temperature of 24 degrees.

There is also a chance of a thunderstorm in the afternoon and evening.

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On Friday there is a 100 per cent chance of rain, with between 25mm and 40mm of rain anticipated.

The temperature will drop to between 17 and 19 degrees.

There will be winds of up to 50km/h and will decrease in the evening.

Over the weekend there is still a high chance of rain but only four millimetres and three millimetres of rain expected on Saturday and Sunday respectively.

Showers are expected into early next week.

On Wednesday, the Bureau of Meteorology's Jane Golding said parts of the state were experience flooding, especially in the Riverina.

"What we are expecting over the next few days time is a low pressure system to deepen over inland NSW and progress toward the eastern part of NSW," she said.

"As we move through Thursday and Friday it will progress towards the coast.

"The catchments are saturated, the dams are full or near full so conditions are really primed for flooding to continue in areas already flooding and to extend to new areas."

Ms Golding recommended people continue to watch and adapt to warnings issued by BOM over coming days.

The bureau has issued a warning for severe thunderstorms, which are likely to produce heavy rainfall that could lead to flash flooding on Wednesday night.

Locations that might be affected include Penrith, Bowral, Campbelltown, Springwood, Bilpin and Nerriga.

NSW SES Commissioner Carlene York said residents should know their properties' flood risk and prepare.

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