If the ACT receives the Bureau of Meteorology's maximum rain forecast of 45 millimetres on Thursday, it would be the city's wettest September on record by far.

As of Tuesday afternoon, Canberra Airport had copped 117 millimetres this September.

Weatherzone meteorologist Kim Westcott said another 34 more millimetres would need to fall over the next three days to beat the record of 150.6 millimetres, set in 1978.

With the Bureau forecasting between 30 and 45 millimetres of rain on Thursday and up to 15 millimetres on Friday, it could pass the mark with flying colours.

"It looks like they're be the heaviest rain on Thursday and we'll see some lingering showers on Friday," Ms Westcott said.

"As Canberra Airport recorded 54 millimetres on Saturday the third of September and the 24 millimetres on the 22nd, it's ranked as the second wettest September so far and it has a good chance of being the first."

This month's wet weather has already more than doubled September's average rainfall of 54 millimetres.

Ms Westcott said the low pressure trough would bring a cold front across NSW and the ACT on Thursday that would also bring some "quite dusty wind".

Wednesday is forecast to be mostly sunny with a top of 18 degrees.

But just when Canberrans will have been getting used to some lovely spring days, the cold snap will drop top temperatures to 12 degrees on Thursday and 10 degrees on Friday, before climbing back up to 21 degrees on Sunday.

"On Thursday, the wind will make it feel a fair bit colder," Ms Westcott said.

Gusts of up to 25km/h were tipped for the afternoon and evening.

And people should prepare to rug up, with night time lows of two degrees forecast on Friday, four degrees on Saturday and three degrees on Sunday.

After the shower or two expected for Friday and Saturday, the rest of spring is also tipped to be wetter than average.

It follows Canberra's third wettest winter on record.

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