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

Thunderstorms are tipped to last until Tuesday evening with up to 70mm of rain forecast to drench Sydney.

Scattered showers are expected across the city 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," he 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."

More than 100mm of rain is due to hit Inverell.

The Bureau of Meteorology on Monday issued a severe thunderstorm warning for heavy rain, large hailstones and damaging winds for a region stretching from Griffiths in the Riverina all the way up to the Queensland border.

Among locations affected include Mudgee, Tamworth, Gunnedah, Moree, Narrabri, Dubbo, Parkes, Condobolin, Coonamble, Nyngan and West Wyalong.

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," he 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."

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