BoM meteorologist Dean Narramore said the Snowy River is "of particular concern" in both NSW and Victoria during his severe weather update on Friday.

"We've got major flooding expected along there [the Snowy River], so residents and communities along this river need to play close attention to the latest forecasts and warnings there," Mr Narramore said.

The Snowy River catchment has already received rainfall totals of 25-50mm in the 24 hours until 11am on Friday and is forecast for another 20mm later today.

Moderate flooding is currently occurring at Buchan while there is minor flooding at Bombala River, McKillops Bridge and downstream from Basin Creek.

Major flooding is possible downstream from Basin Creek to Orbost on Saturday, warned the BoM.

There is also flooding at Mogo and Nelligen on the NSW South Coast, which has been captured on camera.

Meanwhile, Sydneysiders rushed under cover as it hailed during a severe thunderstorm at 11am on Friday.

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Mr Narramore said river levels were responding to heavy rainfall in both NSW and Victoria on Thursday night.

"We saw a lot of rain overnight through parts of southern NSW with widespread areas of 50-100mm through East Gippsland and the South Coast of NSW. We even saw isolated falls in excess of 200mm, just inland of Moruya," he said.

"And then since nine o'clock this morning, we've continued to see light to moderate falls of around 15 to 25mm and that will continue for the rest of today. Because of all this rainfall, we have numerous flood warnings current."

There is a major flood warning for the Weir River in NSW and minor to major warnings for the Barwon, Lachlan and Macquarie Rivers as well as a moderate to major warning for the Moruya and Deua Rivers in the state.

There are also a number of minor and moderate warnings for other rivers in both NSW and Victoria, which can be viewed on the BoM website.

Mr Narramore said heavy rainfall is likely to move from south-eastern NSW and eastern Victoria to central NSW later in the day on Friday.

"You can see the band of the heaviest rainfall quickly moving offshore tonight and clearing the South Coast of NSW and East Gippsland with just a few light showers remaining on Saturday and Sunday," he said.