The downpour came with its own risks - officials warned of flash floods and landslides across New South Wales (NSW), Australia's most populous state - but was generally greeted jubilantly after months of devastating bushfires.

"There's lots of smiles around the place," NSW Rural Fire Services (RFS) Commissioner Shane Fitzsimmons, who has been more accustomed in recent weeks to delivering fire evacuation orders, told reporters in Sydney.

"It is breaking the back of this fire season, no doubt," Fitzsimmons said. "The rain is good for business and farms as well as being really good for quenching some of these fires we've been dealing with for many, many months."

The deluge had the effect of calming the number of active fires in NSW by 20 in a single day, a success rate that NSW RFS said it was "over the moon" about.

By afternoon, there were 40 active fires in NSW, less than half the number of blazes at the peak of the crisis, with all burning at the lowest level danger warning of "advice" only. Neighbouring Victoria state had 21 active fires, also at the lowest level.

Around 11.7 million hectares of Australian wilderness have been razed by a series of huge wildfires since September that has also killed 33 people and more than a billion animals and destroyed thousands of homes. The horrific extended bushfire season has followed a three-year drought across the country.

The heavy rainfall across NSW and Victoria state on Friday was something of a welcome surprise after the Bureau of Meteorology's three-month forecast issued in January forecast continued high temperatures and little rainfall.

Sydney alone was expected to receive as much as 130 millimeters of rain in the 24 hours to Saturday morning, the biggest one-day rainfall since Feb. 5, 2002, data on the bureau's website showed on Friday.

"What we are broadly expecting over the next couple of days is this coastal trough to hang around, to keep delivering this widespread, prolonged, steady rainfall," BOM forecaster Jane Golding said.

Fitzsimmons said he was optimistic the continued rainfall over the coming days would help firefighters in NSW get the 17 blazes that are still categorized as "uncontained" under control.

"We expect rainfall to continue to fall across a lot of these fire grounds and that will result in a number of those being declared contained and hopefully we will be confident we won't see new ignitions from anywhere in those fire grounds," he said.

REUTERS