Desperately-needed rain forecast for NSW bushfire zones also brings the risk of landslides and flooding across scorched tracts of land, authorities have warned.
Firefighters have welcomed the forecast as the best news in months but the Bureau of Meteorology said the rain could be a "double-edged sword".
"Hopefully some of this heavy rainfall will fall over fire sites and help control or even extinguish fires," meteorologist Sarah Scully said on Tuesday.
"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 land slips and trees coming down."
Accumulated totals of 30 to 80 millimetres are forecast, with strong falls possible for fire grounds in the Snowy Mountains, southwest of Sydney and South Coast regions. The rain will likely persist from Thursday until the weekend.
"Over the last week we've seen a real shift in the weather pattern," Ms Scully said.
"From the hot and dry conditions that have unfortunately become all too familiar over the last few months, to much more humid conditions."
Some areas with thunderstorms could receive up to 100mm.
"From a fire point of view, this is the best week we've seen for many, many months in terms of easing some of the conditions," Rural Fire Service commissioner Shane Fitzsimmons told the Nine Network on Tuesday.
The downside is there could be localised flash-flooding, significant erosion and problematic run-off into water catchments.
Mr Fitzsimmons said to protect drinking supplies "filtration and other equipment is being deployed into the water systems".
NSW Water Minister Melinda Pavey said the government was using silt curtains to stop heavy rainfall washing ash into Warragamba Dam.
"Our priority is protecting Sydney's drinking water supplies," she said in a statement.
"The recent fires threatened significant water infrastructure at Warragamba. While much of the catchment has been affected by fire, work by WaterNSW along with the RFS helped prevent damage to the infrastructure."
The fire service has been working closely with the SES, NSW Police and the Bureau of Meteorology to track the location of the heaviest falls.
So far this bushfire season some 2176 homes have been destroyed across NSW. Of that number, 1260 have been lost in the past fortnight.
"This figure is likely to increase as (assessment) teams continue to work through fire-affected areas," the RFS tweeted on Tuesday.
There are still more than 100 fires burning across NSW.
A February concert for Australian bushfire relief, meanwhile, has sold out, with more than 70,000 people to attend "Fire Fight Australia" in Sydney.
The concert will be headlined by rock band Queen while Alice Cooper, k.d. Lang, Delta Goodrem, Jessica Mauboy, John Farnham and Tina Arena will perform.
All proceeds will go to Red Cross disaster recovery efforts.