Millions of Australians have been warned of wild winds, extreme temps and fire dangers on Wednesday, as a record-breaking outback heatwave swoops across the country.

Victoria and South Australia are expected to be slammed by dangerous and destructive winds, while NSW and Queensland will cop searing heat.

Sydney is forecast to swelter through an expected top of 39 degrees on Wednesday, with Newcastle (39 degrees) and Wollongong (36) also in the firing line.

Senior meteorologist Angus Hines said the hot conditions stemmed from a “bubble of heat” stretching across central Australia.

Queensland, NSW and South Australia posted their hottest October days on record on Tuesday, with the mercury hitting 46.1 degrees in Birdsville, 44.8 degrees in Bourke and 45 degrees at Moomba Airport.

The all-time temperatures were delivering heatwave conditions and elevated fire danger, Hines said.

Total fire bans have been declared for the regions of Greater Sydney, Greater Hunter, Illawarra/Shoalhaven, Upper Central West Plains and North Western.

NSW authorities warned walking tracks and remote campgrounds in national parks could be closed “at short notice” if conditions worsen on Wednesday.

Relief from the heat is expected to arrive across NSW and southern Australia on Thursday. But those in the north won’t be so lucky, with temperatures tipped to remain high on Friday and into the weekend.

It won’t be quite as hot in Darwin (35 degrees), Canberra (31), Brisbane (30) and Melbourne (23) on Wednesday.

Melburnians and country Victorians have warned of destructive winds and a risk of thunderstorm asthma conditions.

Victorian Emergency Management Commissioner Tim Wiebusch said damaging winds would hit the state’s west on Wednesday morning and reach the east in the afternoon.

“We’re asking Victorians to ensure that they are prepared for power outages,” he said.

“That means be aware of fallen powerlines that may be on the ground but also ensure that you’ve charged your mobile devices.”

The Bureau of Meteorology has warned gusts of 90-110km/h are expected to be “widespread” and could rise to 120km/h in the south-west Coast, Bellarine and Mornington peninsulas and Bass Coast.

Trees or branches were “very likely” to come down in the wild weather, creating hazards on the roads and properties, Geb Abbott from Victoria State Emergency Service said.

She urged motorists to consider alternate routes before hitting the road, and to consider if they needed to travel at all.

-AAP