Boxing Day travellers are being urged to reconsider their plans, with much of Victoria facing extreme and unpredictable bushfire risks and other states bracing for outbreaks.
Catastrophic conditions are forecast in Victoria on Thursday and Friday, with temperatures likely to reach 40C.
"Our state will have all areas ... in extreme fire danger rating, except for East Gippsland on Thursday," Emergency Management Commissioner Rick Nugent said on Monday.
"Please only travel through these areas if you have to."
Across the country millions of people are being warned to stay alert as the risk of Christmas-week outbreaks intensify.
Several states are battling dangerous blazes or warning of potential threats, with a significant part of NSW on high fire alert.
South Australia's Country Fire Service issued a Leave Now alert with an out-of-control blaze burning at Onkaparinga Hills near Woodcroft in the southern Mount Lofty Ranges on Monday evening.
Regional areas in the state are expecting hot conditions on Boxing Day, with Adelaide facing a peak of 36C after a predicted 37C on Christmas Day.
High to extreme fire danger remains across parts of WA but conditions are forecast to east slightly in coming days.
The most concern is in Victoria, where strong, dry winds of up to 100km/h are forecast to filter throughout from the northwest, extending danger into Friday.
"These conditions will make it far easier for fires to start and to spread and for existing fires to race off in the direction of the wind," Mr Nugent said.
Country Fire Authority chief executive Jason Heffernan warned single-digit moisture levels in the air mixed with the bone-dry bush were a recipe for disaster.
"It is ready to burn," he said.
Heavy rain in Melbourne has done little to quell major fires burning across 41,000 hectares in the Grampians in Victoria's west which is being tackled by 18 aircraft and about 300 firefighters.
"It's a lot of fire in the landscape and when that weather changes on body, that fire is going to move and it's going to move rapidly," Forest Fire Management Victoria chief fire officer Chris Hardman said.
CFA Chief Officer Jason Heffernan says conditions in Victoria could end in disaster. (Joel Carrett/AAP PHOTOS)
Another blaze in Bullengarook, northwest of Melbourne, has burned more than 144 hectares and is out of control with the potential to spread into Ballarat's northern suburbs on Thursday.
Several sheds and outbuildings have been destroyed in another fire at the Gurdies in western Gippsland, with Mr Hardman warning it had "enormous potential" to grow on Boxing Day because of strong winds.
"Be very clear of where you are in proximity to those fires," Mr Hardman said.
Deputy Premier Ben Carroll urged people to continue to heed warnings.
"This is the beginning of what will be a long, hot and dangerous bush fire season," he said.
Ballarat will transform into base camp for about 200 firefighters as 100 personnel from NSW, the ACT, Queensland and Tasmania arrive to help.
Victorian firefighters are preparing for catastrophic and extreme conditions on Boxing Day. (James Ross/AAP PHOTOS)
Mr Heffernan urged those who needed to travel to have the VicEmergency app, a plan on where they were going and up-to-date information.
"If you are thinking of travelling for the Christmas break and you can avoid doing so and it is safe to do so, then perhaps that is a consideration you should give," he said.
The federal government is working on activating disaster recovery payments.