By William Ton and Melissa Meehan

Fire crews are facing tricky conditions amid extreme fire conditions in Australia’s southeast, as residents swelter through the third day of a major heatwave.

The hot and dry weather is impacting areas from central Australia to the southeast on Sunday, with pockets in Victoria’s east and NSW’s southeast experiencing severe heat.

Conditions are set to become more “unsettled” as winds begin to shift and clouds move in, Victoria State Control Centre spokeswoman Reegan Key said.

“People will notice the cloud cover today a bit more, and that creates more tricky conditions for firefighters,” Ms Key told AAP on Sunday.

“Where we already have fires in the landscape, sometimes the winds can be a bit difficult, but equally there is potential for lightning across the state so we might see some new starts.”

An extreme fire danger day has been declared in the Mallee, Central and North Central regions and high fire danger in all other parts of Victoria.

Total fire bans are in place for the districts of Mallee, Wimmera, South West, North Central and Central, including Melbourne and Geelong, which means no fires can be lit in the open air.

CFA Chief Officer Jason Heffernan said the total fire ban had been declared because potential hot and gusty conditions posed a big risk as fires continue to burn across the state.

Firefighters remain on high alert in Victoria as the Grampians National Park fire, which began mid-December, continues to burn out of control but at a slower rate.

More than 76,000 hectares of the park – equivalent to the size of Singapore – have burnt and four houses and several other buildings have been destroyed.

Crews have been working to contain the fire, and the possibility of residents and tourists being able to return to Halls Gap and surrounding areas is rising.

“If we can get through today, they are likely to declare that fire contained in the next couple of days,” Ms Key said.

A heatwave warning for SA, NSW, Victoria and Tasmania was issued by the Bureau of Meteorology, with temperatures set to reach the 40s in some parts of the country on Sunday.

Meteorologist Miriam Bradbury said the mercury is set to push the high 30s to low 40s across Victoria and above average temperatures for South Australia and Tasmania before heatwave conditions move into NSW.

Conditions should begin to ease on Monday for South Australia, Tasmania and Victoria, but NSW will have to wait until Tuesday for any relief.

In NSW, fire crews were battling 46 fires across the state on Sunday amid concerns possible thunderstorms could make matters worse.

Some of those fires are burning in or around the Yengo and Wollemi National Parks and near the Attunga State Forest.

Campers must ensure they put out campfires before leaving the site after firefighters responded to 50 unattended fires this year, Forest Fire Management Victoria Commander Dave Sayce said.

Those looking to escape the heat are urged to exercise water safety after Life Saving Victoria conducted 145 rescues on Saturday.

“That’s a really high number and, obviously, there are lots of people on holiday at this time of year,” Ms Key said.

“We’re asking people to be a bit more aware and careful as they head to the beach, rivers and lakes.”