The Bureau of Meteorology issued multiple severe storm warnings on Tuesday afternoon, with damaging winds, large hailstones and heavy rain expected to hit in several areas.

A severe thunderstorm reported at 4.30pm near Richmond in northwest Sydney was expected to move east with Wilberforce and Cattai in its path.

A low pressure system in the south and a trough in the north generated thunderstorms in other parts of the state too, with severe storms detected forming around the Northern Rivers and the upper Mid North Coast regions.

The BOM on Monday warned severe storms could form and threaten heavy rainfall in a short period of time, which could lead to flash flooding in some areas.

"Storms by their nature, they're pretty hit and miss so not everywhere," BOM senior meteorologist Jane Golding said on Monday.

Damaging winds up to 90km/h are possible, and with many catchments already saturated gusts could bring down trees quite easily, Ms Golding said.

Storm season kicks off officially on October 1 in NSW, Ms Golding said, meaning the latest band of storms is not unusual for this time of year, however conditions in the Pacific and Indian oceans, and the recent declaration of a third La Nina system are also influencing the weather.

The South Coast has a wet Wednesday forecast, while Thursday is expected to be the wettest day of the week in Sydney, before showers ease but continue into the weekend.

Major flooding is occurring in central west NSW at Wee Waa, Warren, Euabalong and Condobolin, expected to continue in the coming days.

Major flood warnings are in place on the Namoi, Macquarie, Bogan, Lachlan, Culgoa, Birrie, Bokhara and Narran Rivers.

A low pressure system is expected to move off the coast on Thursday but showers are forecast to continue into the weekend.

Australian Associated Press