The combination of a low pressure system and associated troughs will bring rain to inland Victoria and NSW from Friday onwards, Tom Hough, a meteorologist from Weatherzone, said.
Thunderstorms are possible before then, with storms a chance for Tuesday afternoon in Sydney. They may return on Saturday when 10-20 millimetres of rain is forecast.
"There's a fair amount of instability and moisture around," Mr Hough said, adding that localised falls could reach 60 millimetres or more on Saturday for areas near Canberra.
"Rainfall rates could be quite intense," said James Taylor, a senior meteorologist with the Bureau of Meteorology's extreme weather desk. "There's some risk of riverine flooding."
"To get a rainfall event like this at any time of the year of the year is certainly unusual," Mr Taylor said, adding that spring is the most likely season for such weather.
For now, the heaviest falls are expected in north-eastern Victoria, and the south-east inland areas of NSW, such as the Southern Tablelands and the Riverina, Mr Taylor said.
The latest model runs are paring back the rainfall totals for regions such as Sydney, with mostly showers and the chance of a thunderstorm predicted for Friday and Saturday at this stage.
Still, those planning outdoor activities this week will probably want to pack an umbrella, particularly if they are in the alpine regions, and keep an eye out for thunderstorm warnings. (See eight-day rainfall chart below from the Bureau of Meteorology.)
Hot and cool
A large blocking high pressure system in the Tasman Sea has kept temperatures on the cool side for much of eastern Australia but brought much more humid and warm conditions further south.
"We've had tropical moisture broadly across the continent for several weeks now," Mr Taylor said.
Sydney, for instance, will come close to posting its first below-average month for maximum temperatures in almost six years. A slightly warmer than usual end to November, though, may keep the streak alive.
The tale is different for sites further south, with Melbourne and Hobart on course for their warmest Novembers on record.
"The November heat is a record in multiple regions," Mr Taylor said.
The Victorian capital had nine days in a row of at least 28 degrees, beating the previous record of six such days, Mr Hough said.
A couple of days in the mid-30s are forecast to end the month.
Hobart will also end November with a burst of warmth, with hot and humid conditions tipped for Thursday and a top of 33 degrees - warmer than Cairns in far north Queensland is expecting for any day in the coming week.
Instability, though, is starting to develop further north, bringing more thunderstorms to NSW on Tuesday.
#Thunderstorms possible across much of the state today. #Severe thunderstorms likely about the Upper Western as a surface trough and humid airmass combine. Heavy rain and damaging wind gusts are possible. Keep an eye on the latest warnings at https://t.co/JEtz9REW28pic.twitter.com/XuXkQDuXNx??? Bureau of Meteorology Australian Capital Territory (@BOM_ACT) November 27, 2017
The coming rain event may break up the weather patterns that have dominated the country's south-east of late.
"This system looks like it is clearing things out," Mr Taylor said.
What happens after then remains unclear, with one possibility that another blocking high sets up in the Tasman.
Still, the coming soaking rains should ease back the fire risk in Victoria and southern NSW that was already looking likely to moderate as a weak La Nina takes hold in the Pacific.
"It will definitely help push things back somewhat," Mr Taylor said.
Fire authorities, though, last week stressed that the dry run through much of winter and into spring meant an above-average fire season is still expected for most of Victoria and a large swathe of NSW including Sydney.
Weatherzone is owned by Fairfax Media, publisher of this website.
The story 'Quite intense': Nation's south-east set for unusually heavy soaking first appeared on The Sydney Morning Herald.