Waves of "ferocious" thunderstorms are expected to lash NSW on Monday, with flash flooding, heavy downpours and "giant hail" likely to hit large parts of central and northern New South Wales.

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 south Australia. 

When combined, the dynamic conditions could trigger "some of the worst thunderstorms of the season", Weatherzone Meteorologist Rob Sharpe said on Sunday. 

​"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 5 centimetres in diameter," he said.

"The most at risk area is near a line from Parkes to Moree." 

The worst of the weather is expected to hit Sydney on Monday afternoon, with thunderstorms tipped to last until Tuesday evening and scattered showers expected across the city for the rest of the week.

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

On Sunday morning a rock fisherman died after being swept from rocks without a life jacket at Manly on Sydney's northern beaches. 

A top of 27 degrees is forecast for the city on Monday and 29 degrees for the west.

Maximum temperatures in the mid 20s are forecast for each day in Sydney for the rest of the week.

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