WaterNSW has installed giant screens to clean bushfire ash out of water supplies.

Heavy rainfalls are predicted for parts of the drought-affected state this week, including on fire grounds in the Snowy Mountains, southwest of Sydney and South Coast regions.

WaterNSW is preparing to limit potential impacts on the water quality in dam storages caused by ash run off, including using silt curtains to stop ash being washed into Warragamba Dam.

The authority has an additional 1000m of silt curtains in case they are needed in either Nepean or Tallowa Dams.

WaterNSW chief executive David Harris says filtration and other equipment is being deployed too.

The Bureau of Meteorology said the rain could be both good and bad news.

“Hopefully some of this heavy rainfall will fall over fire sites and help control or even extinguish fires,” meteorologist Sarah Scully said this week.

“But it's a bit of a double-edged sword because heavy rainfall and gusty thunderstorms bring the potential for flash flooding, particularly in the burnt-out areas of NSW and Victoria which are now vulnerable to landslips and trees coming down.”