Severe thunderstorms are likely to produce damaging winds, large hailstones and heavy rainfall that may lead to flash flooding in the warning area, which now covers the entire region, over the next several hours.

Locations which may be affected include Wollongong, Nowra, Batemans Bay, Gosford, Sydney, Armidale, Orange, Tamworth, Dubbo, Parkes and Lightning Ridge. 

The State Emergency Service advises that people should: 

The next warning is due to be issued by 7.50 pm.

4.40pm: A severe weather warning has been issued for the northern most part of the Illawarra. 

Severe thunderstorms are likely to produce large hailstones, heavy rainfall that may lead to flash flooding and damaging winds in the warning area over the next several hours.

Locations which may be affected include Gosford, Sydney, Armidale, Orange, Tamworth, Gunnedah, Moree, Narrabri, Walgett, Dubbo, Parkes and Lightning Ridge. 

The next warning is due to be issued by 7:25 pm.

If severe thunderstorms develop in Wollongong or surrounding areas, a more detailed warning will be issued.

Visit the Bureau's website for more information. 

Earlier: Waves of “ferocious” thunderstorms are expected to lash NSW on Monday, with flash flooding, heavy downpours and “giant hail” likely to hit central and northern parts of the state.

The storms are expected to move south, hitting Sydney and the Illawarra on Monday afternoon.

Thunderstorms are tipped to last until Tuesday evening with up to 50mm of rain forecast to drench Wollongong.

The Bureau of Meteorology said a thunderstorm was likely in Wollongong on Monday afternoon and evening, with a high chance of showers also later in the day.

Tuesday will bring with it a very high chance of showers in the city and the chance of a thunderstorm, with locally heavy falls, the bureau said.

Scattered showers are expected across Wollongong for the rest of the week.

A line of storms stretching from Broken Hill, Echuca and the Victorian town of Port Albert will be joined by storms that are moving along a low and upper-pressure trough across the eastern parts of southern Australia. 

When combined, the dynamic conditions could trigger “some of the worst thunderstorms of the season”, Weatherzone meteorologist Rob Sharpe said. 

“The worst storms will be spread across the North West and Central West Slopes and Plains districts in NSW,” Mr Sharpe said.

“Storms in these districts are likely to see large hail and have a significant risk of producing damaging wind gusts above 90km/h and giant hail greater than five centimetres in diameter.

“The most at-risk area is near a line from Parkes to Moree.”

Powerful surf conditions will continue to batter most of the NSW coast, potentially creating hazardous conditions for rock fishers, surfers and swimmers. 

NSW SES spokesman Phil Campbell said on Monday that emergency services were “quite concerned” about the “supercell thunderstorms”.

"We're asking people up in that north-east part of the state and also inland around Tamworth and Moree just to make sure they're well prepared,” Mr Campbell said.

“At the moment, we've not had any calls for assistance, which is good news.

“We do have a number of weather models, according to the bureau, that are forecasting very heavy rain from the middle to late part of the week. We're just keeping an eye on that.”