In Wollongong, as much as 40mm could fall between Thursday and Monday, while Nowra could get up to 60mm of rain.

Parts of the Southern Highlands, such as Bowral could get even wetter, with the possibility of as much as 90mm falling - 40mm of it on Thursday alone.

"It is one of the most positive forecasts we've seen for some time," said Rural Fire Service NSW Inspector Ben Shepherd.

"By no means is it drought-breaking, but this will enable so many of our firefighters to take a well-earned break.

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"It will basically see a halt to any backburning operations and it will basically hold the fires in their positions, depending on how much we get and where we get it."

He added that early forecasts had some areas expecting as much as 100mm.

Insp Shepherd said there was a chance that the falls could be so big as to extinguish some of the fires - particularly in the north of the state.

"We'll have to wait and see what it does to some of these other fires, particularly in the Kosciuszko National Park but many of the coastal fires should get a considerable amount of rain and we may even be able to say they are contained," he said.

Thunderstorms were also predicted, though if they brought lighting strikes he said the risk of new fires starting would be small.

"While there is talk of possible thunderstorm activity any lightning that may occur as a result of those storms is likely to be accompanied by rainfall anyway," he said.

"So there's less risk of any new fires given the forecasted rain."