Widespread late-spring falls are predicted to soak much of Australia's south-east in coming days, with conditions described as "tropical" in areas used to much milder conditions.

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.