Thunderstorms have lashed NSW from the north to the south coast, and the state will enjoy only a brief respite before the rain falls again.

Widespread thunderstorm activity increased Monday, with between 40mm and 50mm of rain falling across a broad area and some places copping an even larger drenching.

But as the week progresses, a trough over the western part of the state is likely to move eastward.

Friday and Saturday should bring widespread rain and thunderstorms moving across the state, Ms Reid said in a statement .

The atmosphere in NSW has been humid for the past week, with the moisture coming down as rainfall.

The state is in a La Nina weather pattern, which means high humidity and above average rainfall. The La Nina pattern is expected to continue at least to early autumn.