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.

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.