Whether returning from a refreshing coast trip or having soaked up some sun in your own backyard, you'll need to swap your sunnies and hats for raincoats and umbrellas as Canberra is set to see about 20 millimetres of rain in the next week.

That's more than half the amount of rain that fell throughout December (35 millimetres).

The showers could of course be heavier or lighter than tipped, but they should come through late on Saturday and likely reappear in patches until at least Thursday, said Weatherzone meteorologist Kim Westcott.

"It will hang around in and out on Sunday, maybe Monday and becoming patchier by Wednesday," she said.

"It's a pretty dynamic system we've got at the moment which will affect NSW and Canberra … we've got a big low pressure trough that's building up moisture."

It could also bring in some thunderstorms.

The wet weather and cloudy skies are set to break up the hot spell the ACT has seen in the past few days, with maximum temperatures forecast to stay within the 20s in the coming week.

Saturday is tipped to reach a top of 29 degrees, followed by tops of 21 on Monday, 20 on Tuesday and mid-20s by Thursday.

"We will see temperatures become much more cooler and nicer, for some people, from around Sunday," Ms Westcott said.

"There are no indications of strong winds; it is looking fairly calm on the wind front."

Canberra is coming out of quite a warm December – maximum temperatures were about three degrees above average – but Ms Westcott said she was unable to tell how hot January will be.

"Generally for January, Canberra can expect daytime temperatures averaging around 28 degrees," she said.

An El Nino pattern is responsible for Canberra's summer being hotter and dryer than average so far.

It lets heat built over north-west Australia, then as troughs build across the country, the hot air drags down to south-east Australia and affects Canberra.