"It feels like a little blast of winter," forecaster Gabrielle Woodhouse said.

The gloomy outlook is for 30 to 50 millimetres of rain to fall in the next day or so, with the greatest drenching concentrated on Thursday night.

The broad pattern is heavy rain overnight, clearing up somewhat on Friday morning but then with showers starting again on Friday afternoon.

And there will be no let up through the weekend and into early next week.

Apart from the rain, temperatures are expected to plummet, according to the bureau.

"The main thing for the weekend is that it's going to be quite wintry," the meteorologist said.

The overnight minimum on Tuesday is forecast to be two degrees. Some higher ground could see frost. The maximum on Sunday may be as low as 12 degrees, but feel colder because of wind.

There is a chance of the tops of the Brindabellas turning white.

The wintry weather for Canberra is mirrored across the region.

Flooding was expected as large parts of NSW with a month's worth of what the bureau calls "intense rainfall" forecast in the coming days.

The bureau said there were two dozen catchments across the state in danger of minor to major flooding into the weekend.

Meteorologist Jane Golding said some areas could receive a month's worth of rain before the end of the week.

"We're seeing rainfall records broken that we haven't seen in 10 years," she said.

The state's emergency services also warned of flooding. SES assistant commissioner Dean Storey said the service had gone into a "heightened state of readiness".

"The potential impact area for this weather event is quite vast." He said many roads would be cut off.

The cause of the wintry weather was because a "very moist and unstable air mass extends across the state" as a low-pressure trough in South Australia combined with a tropical air mass coming from the north.

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