A flash flooding warning has been issued for Goulburn, the Illawarra and parts of the South Coast.
The regions are expected to receive between 100 and 150 millimetres of rain in the next 24 hours, with some areas possibly getting up to 250.
The severe weather warning stretched from Wollongong to Crookwell, and south towards Braidwood and Ulladulla.
In Canberra, the downpours are likely to be lighter at 10 to 15 millimetres.
The capital had recorded 16 millimetres of rain at the airport by 7am on June 6.
Towns on the South Coast, including Jervis Bay and Ulladulla, had already recorded 96 and 68 millimetres respectively.
Forecaster Christie Johnson said with the potential of more than 100 millimetres of rain in some places, they are expecting to see flood impacts.
"We are expecting to see river rises in catchments," she said.
"We will start issuing flood warnings as we see the rain starting to fall the and rivers starting to rise in response.
"We also have particularly high tides along the coast this week, and so combined with this heavy rainfall we could see minor flooding in some of our tidal rivers as well."
The bureau reported that autumn had been warmer than usual for much of Australia, with the mean temperature around 0.5 degrees above the 1961-1990 average.
Autumn has been wetter than usual in some parts of Australia, with overall rainfall 26 per cent above the 1961-1990 average.
Almost all of Australia, including Canberra, was very likely to experience another "unusually warm" winter, according to the Bureau official outlook for the season.
Last year was Australia's warmest winter on record. Some meteorologists believe this year could be even warmer in some places.
