By Rachael Ward and William Ton in Melbourne

Extreme weather conditions have firefighters on high alert as southern Australia swelters through a three-day heatwave and sticky conditions push the mercury into the 40s. 

Hot conditions are expected to continue on Sunday from central Australia to the southeast, with pockets in Victoria’s east and NSW’s southeast to experience severe conditions until late Sunday.

As parts of the state are on alert for fire, hundreds of holidaymakers headed to the beach to escape the hot conditions.

Authorities are monitoring fire threats in two of Victoria’s most popular national park holiday destinations on opposite sides of the state, with tourists and locals in the Grampians and Wilsons Promontory urged to monitor alerts.

“The fire risk actually is escalating and we’re just looking at some of the numbers for extreme fire risk, especially around greater Melbourne,” State Control Centre spokeswoman Reegan Key told AAP on Saturday.

“We’re likely to have extreme fire risk around greater Melbourne tomorrow, also in the Mallee and the Wimmera.”

Total fire bans have been declared for the Mallee, Wimmera, Southwest, Central, and North Central districts on Sunday.

“Tomorrow’s conditions will make it difficult for firefighters to suppress a fire should one start,” CFA Chief Officer Jason Heffernan said on Saturday.

“Especially with fires in the landscape already, we’re asking people to follow the strict conditions associated with the total fire ban declaration.”

The Bureau of Meteorology issued a heatwave alert for the state’s north, central areas and east, with an extreme alert for far-eastern Gippsland including Mallacoota.

The Grampians National Park bushfire has slowed but continues to burn, with four houses and several other buildings lost in recent weeks.

A bushfire at Sandy Point Yanakie at the entrance to Wilsons Promontory that started on Friday was at advice level on Saturday, with authorities stating there was no threat to communities although the two-hectare blaze was not yet under control.

“Firefighters have managed to stop the spread of the fire in a sensitive area and will remain on scene,” the Vic Emergency website said.

More than 76,000 hectares of the Grampians – equivalent to the size of Singapore – have been burnt.

There are plans to allow locals and visitors back into Halls Gap in coming days but Ms Key said the call was a “balancing act” that would depend on conditions across the weekend.

Forest Fire Management Victoria State Agency Commander Dave Sayce urged everyone to do their part to prevent bushfires.

“If you’ve had a campfire last night, make sure it is properly extinguished by now, using water not soil,” he said.

High fire danger warnings have been issued for most of South Australia on Saturday with the Mount Lofty Ranges, which borders Adelaide, in extreme fire danger.

The mercury is tipped to reach the mid to high 30s in southern districts, rising to low 40s in inland areas.

A cool change is due to sweep through SA, Victoria and Tasmania by Monday.

Heatwave warnings and severe thunderstorm warnings are in place for parts of NSW on Saturday afternoon.