Severe rainfall is forecast to continue in already-saturated NSW as a flood watch is issued for multiple river catchments across the state.

NSW has been hit by repeated flooding in recent months, with the Northern Rivers devastated by two deluges within weeks, and Sydney drenched in its wettest March on record.

A severe weather warning is in place for southern and central NSW, metropolitan Sydney, the Illawarra, the South Coast, the Central and Southern Tablelands and parts of the Hunter.

Heavy rainfall which could lead to flash flooding was forecast to develop on the Illawarra, South Coast and Southern Tablelands over Wednesday night, with falls to extend across Sydney, the Central Tablelands and Hunter region on Thursday.

Six-hourly totals between 60 and 100mm are forecast, with totals of up to 140mm predicted on the coast.

The Bureau of Meteorology warned of an increased risk of landslides caused by heavy rain.

A flood watch has also been issued for central NSW, with minor to moderate flooding forecast for the Southern Coastal Rivers including the Hawkesbury-Nepean, the Macquarie and Queanbeyan rivers on Thursday and Friday.

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Catchments are already soaked after months of heavy rain, and warnings will be issued if flooding eventuates, the BOM said.

The weather is caused by a strong upper trough and embedded low amplifying over the centre of NSW, working to deepen another trough sitting off the coast.

The systems are expected to weaken on Friday morning.

“Heavy and persistent showers over the coming days will increase the chance of flash flooding and landslips over already saturated catchments,” BOM meteorologist Sarah Scully said on Wednesday.

Severe thunderstorms also pose a threat, including in northeast NSW.

“They may produce localised heavy falls (but) it is not expected to produce that riverine flooding,” Ms Scully said.

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“Instead, it’ll be more localised flash flooding.”

© AAP 2022

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