The Bureau of Meteorology has forecast up to 35mm for Thursday, with the chance of a thunderstorm, while Friday could be wetter still, with up to 40mm.

The biggest thing to watch for ACT residents will be heavy rainfall on the south coast of NSW, the bureau's NSW and ACT manager Agata Imielska said.

"The heaviest rainfall is focused on the far south coast of NSW. So that's important to be across if you're commuting between Canberra and the areas where we are expecting that heavier rainfall," she said.

Intense and, at times, torrential rain as well as strong winds mean "it's very important to be across the potential for flash flooding [and] what are roads and closures".

"Today and tomorrow will be the key days for conditions just to stay safe especially on the roads," Ms Imielska said.

The ACT has experienced 5.4mm of rainfall in December, but it's too soon to say whether another record is on the cards.

IN OTHER NEWS:

Heavy rainfall last month caused the national capital to sail pass the previous record of 138.2mm, set in 1995.

"We are broadly expecting a wetter summer and a wetter December than normal but whether that'll be a record, it's a bit harder to tell," Ms Imielska said.

"There's a fair chance that by the weekend, Canberra ... would have already recorded its December average rainfall, so then it will come down to what we'll see in for the rest of December.

"The record is quite a bit higher than what it was for November - it's 215.2mm for December."

An ACT Emergency Services Agency spokesperson said despite the rain it hadn't been a "significant night" for the SES, with volunteers responding to fallen trees and leaky roofs overnight.

Queanbeyan SES Unit operations manager Brent Hunter said that while the week had been quiet, he was expecting Thursday and Friday to be busy, with Googong Dam at 100 per cent capacity and expected to spill over.

"The risk of flash flooding is ever-present," Mr Hunter said.

"Flash flooding has been a big problem for us over the past few weeks."

Mr Hunter said this was the new reality with the new La Nina weather pattern, and urged caution around floodwaters.

"When it's raining stay away from creeks, rivers and culverts. If you go in, there is a big chance you won't come out," he said, adding that people should never drive through floodwaters.

Mr Hunter said SES volunteers had performed multiple rescues at Briars Sharrow Road on the ACT/NSW border in past weeks.

Our journalists work hard to provide local, up-to-date news to the community. This is how you can continue to access our trusted content: