The Bureau of Meteorology has issued a severe weather warning for a swathe of NSW from Cooma in the south-east to Wentworth in the west and towards Cobar in the north.
The region can expect heavy rainfall, reaching in excess of 100 millimetres during Friday and Saturday. Some parts closer to the ranges near Victoria could collect double that rainfall.
Damaging winds averaging 90km/h and peaking at more than 110km/h are possible over alpine areas from late on Friday.
"This will be a significant rain and flooding event for eastern Australia," Andrea Peace, a bureau meteorologist, said.
The source of the big wet is a stream of moist air that has been directed over the continent for days from a near-stationary high-pressure system in the Tasman Sea.
A trough will combine with a low-pressure system forecast to form over western Victoria on Friday, Ms Peace said.
Sydney will likely dodge most of the wild weather with only the chance of showers on Friday. More rain is expected on Saturday with the chance of a thunderstorm, the bureau said.
The low and trough combination "will weaken as it comes towards Sydney", Angus McLean-Smith, a meteorologist with Weatherzone, said.
Daytime temperatures in Sydney will drop back from high 20s over coming days to the mid 20s from Monday as the system moves out into the Bass Strait and then the Tasman on Sunday.
Victoria is likely to cop the worst of the conditions, with the bureau issuing a severe weather warning for the entire state as heavy rain and thunderstorms begin to move in from late Thursday.
Melbourne, which has posted a record number of days of 30 or more degrees for both November (12 days) and spring (15 days), may get a month's worth of rain or more by Sunday.
On current forecasts, the city may collect 25-70 millimetres of rain on Friday, 30-80mm on Saturday and 20-50mm on Sunday. Melbourne's long-run average December rainfall is 59mm.
In NSW, towns that may be affected by the severe weather include Yass, Condobolin, Wagga Wagga, Albury, Young, West Wyalong, Tumbarumba, Deniliquin, Griffith, Hay, Narrandera and Thredbo, the bureau said.
A separate flood watch for NSW has flooding potential for 13 catchments including the Snowy, and parts of the Murray River. Moderate to major flooding is possible for the central Murrumbidgee River and the Tumut River.
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The story 'Significant' rain to dump flooding rain on inland areas first appeared on The Sydney Morning Herald.