A high-pressure system "as big as Australia" could bring Sydney's warmest patch of weather since   March, while most of the country is likely to go for a week with little rain. 
Sydney's temperature should nudge 22 degrees on Friday before the city heads into what could be four days with tops of 25 degrees, Anthony Duke, a meteorologist with Weatherzone said.
"We haven't had a run of four days of 25 degrees since late   March," Mr Duke said, adding that the timing of the arrival of a cool change on Tuesday afternoon might determine whether the series of warm days was achieved.
The warm days and mild nights - minimums should range from 12-15 degrees - are the result of a dominant high-pressure system that's "about as big as Australia" and is now directing very dry and warm winds from north-western Australia into the country's south-east, Mr Duke said.
With the northern dry season well into its second half, that region is "as dry as it's going to be", he said.
Sea temperatures of about 20 degrees off Sydney are also among the most anomalously warm around Australia, or as much as 2.5 degrees above usual for this time of year, Mr Duke said.
The large high pressure system is now moving into the Tasman Sea but will leave very stable conditions behind for much of the country. 
Almost all of Australia will be dry on Friday, according to the Bureau of Meteorology's forecast:
The dry spell, while welcomed by cricket competitions that get going over the weekend in Sydney and elsewhere, won't be well received by farming communities needing spring rain.
While NSW had reasonable winter falls in many regions, large parts of Victoria and Queensland again missed out.
The coming week should also be dry for most of the nation, according to the eight-day rainfall outlook:
The bureau this week released its streamflow (or river level) forecasts for the   September-  November period and predicted 98 of the 132 locations it tracks across the country can expect relatively low flows during spring.
Cooler conditions will reach Sydney by the middle of next week with a few showers likely and a "worst case" that a Tasman low will form, Mr Duke said.
The low-pressure system, though, is unlikely to hang around and should move off quickly, he said.
Earlier this week, the big high pressure system brought to Perth its warmest day on record for this early in the warming season, with 32.1 degrees reached on Wednesday.
Weatherzone is owned by Fairfax Media, publisher of this website.