Torrential rain has extinguished the number of active fires in eastern Australia by almost a third - in just one day, according to fire officials.

There had been 62 active fires Wednesday night, with 20 not contained, according to New South Wales Rural Fire Service.

It comes as Australia's largest city, Sydney, recorded its wettest day in over 15 months on Friday.

Fire Service commissioner for New South Wales, Shane Fitzsimmons, confirmed this has dropped to 42 active fires in just 24 hours, leaving 17 "out of control".

Officials wrote on Facebook: "Today we were over the moon to see rain arrive across many parts of New South Wales, with decent fall in the State's north."

"Fingers crossed we see this rainfall remain steady and reach the firegrounds in Southern NSW over the weekend," they added.

Some areas were four to six inches (100 to 150 mm) deep in rain and more rain was in the forecast for the southeastern coast of Australia.

The weather comes as a slight relief for some areas dealing with wildfires that devastated large swathes of land across the state over recent months.

Despite this, the Bureau of Meteorology issued a severe wet weather warning along the New South Wales coast - with damaging winds, heavy rainfall and "abnormally high" tides forecast.

Rescue services said they had rescued a number of people trapped in cars amid rising water.

Rescue services said they had rescued a number of people trapped in cars amid rising water. Credit: AP

There have been close to 1,000 calls for help in NSW and Queensland since Wednesday.

A burst sewer also caused a sinkhole in the middle of a motorway bringing heavy traffic and road closures, according to local reports.

The state's northern coastal town of Byron Bay was drenched with 280 millimeters (11 inches) of rain in the last 24 hours, the Bureau of Meteorology said.

The Rural Fire Service commissioner for New South Wales, Shane Fitzsimmons, said he's optimistic the rain will fall over the fire zone over the coming days, which should help contain some blazes.

He added: "This has been an absolute welcome disruption to the weather pattern and a massive reprieve and relief to so many people."

Australia's bushfire season, which began in September, could run until as late as this Spring.