Temperatures are forecast to nudge 50 degrees and break records in coming days, as a scorching heatwave grips much of Australia’s south-east.

The mercury is forecast to soar on Tuesday across Victoria and South Australia, and could last for seven or eight days.

It comes as the seaside town of Ceduna, South Australia, wilted with the nation’s highest temperature in two years on Australia Day — hitting 49.5 degrees at 4.30pm.

Weatherzone says it was the fishing and tourist town’s hottest temperature ever recorded and possibly the equal-highest at a location on Australia’s southern coastline.

On Tuesday, many Victorian towns are forecast to hit 49 degrees for the first time — including Mildura and Ouyen — and some places could nudge 50 degrees, such as Hopetoun, which holds the current state record of 48.8 degrees.

Victoria has never recorded a 49-degree day in data extending back more than a century at many locations, according to Weatherzone.

Melbourne is set to swelter through its hottest day since the devastating Black Saturday bushfires in 2009, with a maximum temperature of 44 degrees expected.

Mildura, in the state’s far north, is forecast to reach 49 degrees, while Broken Hill in NSW is expected to hit 47 degrees, with both locations set to break temperature records.

“If Mildura does reach (that high), it will be the hottest temperature recorded in Victoria,” senior meteorologist Dean Narramore told AAP.

“Pretty much all observation points near the SA and Victorian border are either approaching or likely to exceed January records and a few locations could also break their all-time records.”

It comes as a total fire ban remain in place for both Victoria and SA as several fires burn out of control under dynamic conditions.

Records were broken on Monday in Adelaide where the temperature reached 44.7 degrees, exceeding the city’s previous Australia Day heat record set in 2006.

Ceduna, northwest of the capital, climbed to 49 degrees, surpassing its previous record temperature, while NSW town Dubbo reached 46.1 degrees to record its hottest January day.

The weather conditions are worrying authorities in Victoria as multiple fires continue to burn out of control, including in the Otways where a blaze jumped containment lines at Carlisle River on Saturday, threatening the small community of Gellibrand.

Authorities say there has been unconfirmed property loss within the fire grounds.

Evacuations were advised late Monday, with residents of more than 1000 homes urged to leave before sunset in anticipation of a massive fire spreading and the arrival of a severe heatwave.

Victoria Police and State Emergency Service crews began door knocking homes and properties in the Greater Otways, strongly recommending that residents evacuate immediately.

Those living in the southwestern Victorian towns of Gellibrand, Kawarren, Beech Forest, Forrest, Barongarook and surrounding areas in the Otways National Park are being urged to escape the growing Carlisle River fire.

The bureau warns gusty southwesterly winds will hit Melbourne on Tuesday afternoon, with a risk of thunderstorms producing dry lightning.

A cooler change will begin to move through Victoria on Wednesday, although inland parts of the southeast won’t see much of a reprieve.

Deputy incident controller Alistair Drayton says the gusty change is worrying, with authorities urging residents living in Gellibrand, Kawarren, Beech Forest, Forrest and Barongarook to evacuate.

“The conditions tomorrow mean the fire could spread quickly and unpredictably. We strongly recommend people in affected areas to enact their bushfire plan, and leave early to protect themselves and their families,” he said.

The heatwave conditions have also sparked a warning from paramedics, who responded to 11 cases of children locked in cars over the weekend as temperatures soared into the 40s in Victoria.

Ambulance Victoria urged the community never to leave children, pets, or older people unattended in vehicles.

“The temperature inside a vehicle can double and become deadly within minutes,” director of emergency management Dale Armstrong said.

“It is particularly dangerous for children to be left inside vehicles, as a child’s body temperature rises three to five times faster than an adult.”

Residents in both states are urged to drink plenty of water, check on family, friends, and neighbours to see if they need help and stay indoors.

-with AAP