As of today, the capital's summer average was just 18.7 degrees – two degrees colder than last summer and three degrees colder than in 2010.

While the summer average was below normal, so too were maximum temperatures.

Weather Channel meteorologist Tom Saunders said the average maximum of 25.5 degrees was almost 2 degrees below normal.

More than 200mm of rain had fallen on the capital as of 8am today – well above the summer average of 168.7mm, but below last summer's 382.2mm.

The Bureau of Meteorology said a slow-moving cloud band, which developed overnight across southwestern NSW, was expected to cause widespread rain over the southern half of the state.

Victoria has already seen torrential rain and received a thorough drenching the past two days.

The bureau warned of heavy rain and thunderstorms, which could lead to flash flooding, in the ACT, south coast and Snowy mountains .

"Three day totals from Wednesday through to Friday are expected to be in the range of a further 100 to 300mm within this area," the bureau warned.

Moderate to major flooding is still likely in the Queanbeyan and Molonglo Rivers.