A burst of warmth will spread across southeastern Australia this week, with a relatively brief spike in temperatures in Melbourne, Adelaide, Hobart and Canberra around the first day of the new season, while Sydney will be unseasonably warm all week.

On Tuesday, September 1:

Melbourne is predicted to reach 22°C (average Sept maximum 17.3°C).

In Adelaide, the mercury should also peak around 22°C (average Sept maximum 18.5°C).

Hobart is forecast to reach 19°C but there's a chance the temperature could even tip over the 20-degree mark (average Sept maximum 15.2°C).

Canberra is expected to reach 20°C on Tuesday, with maximums of 21°C expected either side of it, both this Monday and again on Wednesday.

Since this Monday is the last day of August, it's worth noting that twenty-one degrees is an exceptionally warm winter temperature for the national capital, given that Canberra's average August maximum is just 14.4°C.

While Canberra topped the 21-degree mark in August two years ago (21.4°C on August 24, 2024), the previous time it reached 21°C in August was back in 2015.

What will cause the warm start to the season?

Air circulating anticlockwise around the high pressure system centred over the Tasman Sea will drag relatively warm air from Australia's interior across the southeast.

But the warmth will dissipate by Wednesday in Adelaide, Melbourne and Hobart, and by Thursday in Canberra, as cooler air associated with a weakening cold front sweeps in from the southwest.

Image: Mean sea level pressure and 850 hPa temperatures predicted for 4 pm (AEST) on Thursday, September 3, 2026, according to the ECMWF model. Source: Weatherzone.

Sydney to hit high-20s twice this week, largely unaffected by cooler air further south

Of the southeastern capitals, Sydney is the only one that will barely feel the influx of cooler air.

The harbour city is expected to reach 26°C on Tuesday to start spring, 27°C on Wednesday, and 26°C again on Thursday before dipping slightly to 23°C on Friday (average September maximum 20.2°C).

By the weekend, significant early spring warmth returns, with a maximum around 29°C expected across the city from the western suburbs to the coastal fringe. There's even a chance the city could reach 30°C – a mark that hasn't been recorded in the first week of September since 2020.

Dry and sunny for Sydney, showers developing in other SE capitals

Image: Predicted rainfall accumulation across SE Australia to 10 am (AEST) on Thursday, September 3, 2026, according to the ECMWF model. Source: Weatherzone.

Sydney is the only southeastern capital which is expected to remain dry over the coming week.

While Melbourne, Adelaide, Hobart and Canberra should all see at least a few showers across several days as maximums dip back into the teens, Sydney will be protected by the ranges to the west of the city – as often happens in westerly streams in late winter and early spring.

READ MORE: Warm start to spring to follow Sydney’s warmest winter on record