Heavy rainfall is predicted to develop along the South Coast during Wednesday, due to the combination of a low pressure trough and an onshore airstream.

Up to 30 mm of rainfall is expected to hit the Nowra region on Wednesday, with 100 mm predicted for Thursday.

BoM warned minor flooding is possible for Shoalhaven and St Georges Basin river catchments over the next two days.

It comes after Nowra recorded 676 mm of rainfall across 21 days in March, smashing the month's average of 116 mm.

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Flood watch means that people living or working along rivers and streams must monitor the latest weather forecasts and warnings, and be ready to move to higher ground should flooding develop, says BoM.

Official flood warnings will be issued by BoM if minor flood level is expected to be exceeded.

The announcement comes after St Georges Basin residents were ordered to evacuate by NSW SES early March due to flooding from the east coast low.

Read more: Two border collies rescued from St Georges Basin floodwaters

NSW SES assistant commissioner, Dean Storey, urged residents to take steps to prepare their properties now before more rain arrives.

"It is likely parts of NSW including the Central Coast, Sydney, the Illawarra and South Coast will experience heavy rainfall in a short time period," he said.

"This means we will see flash flooding occur where roads could become flooded quite quickly.

"People can help prevent storm damage around their properties by trimming back overhanging branches, cleaning gutters and downpipes and securing or putting away loose items around the yard or balcony."

NSW SES urged drivers to take care on the roads.

"Never drive, ride or walk through floodwaters. It's too dangerous - and you never know how deep the floodwaters are, the condition of the road or how fast the water is flowing," assistant commissioner Storey said.

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