Adelaide, Melbourne and Hobart just registered their warmest day this early in spring for seven years.

A stream of warm northerly winds ahead of an approaching cold front caused temperatures to climb more than 10 degrees above average in parts of southern Australia on Monday.

Image: Forecast surface temperature on Monday afternoon according to the ECMWF-HRES model.

In South Australia, Adelaide had reached 28.8 degrees by 2:29pm, which is 10.4 above the long-term September average and the city's warmest day this early in the season since 2013. Further west, Ceduna had climbed to 37.9 degrees by 3:06pm, its highest September temperature in 30 years.

Melbourne's 22.9 degrees shortly before 4pm and Hobart's 23.2 degrees at 2:13pm were also the warmest days this early in spring for seven years in each city.

The warm air will shift further north and east on Tuesday, causing the mercury to reach the mid to high-twenties in Sydney. However, it's unlikely to be as warm as last Thursday, when the city reached 30.5 degrees.