The blaze in Victoria's Grampians National Park has razed 36,000 hectares and remains out of control despite more than 300 firefighters battling to bring it to heel and secure containment lines before Thursday.
The fire changed direction late on Sunday, prompting evacuation warnings for hamlets east of the park
The same warning came for towns on Perth's outskirts threatened by a fast-moving blaze while Sydneysiders were placed on high alert for fires on Monday.
The Grampians blaze tripled from Friday to Saturday due to high temperatures and strong winds.
Victorian Country Fire Authority deputy chief officer Garry Cook said ground and air crews were working to reduce the spread of the Yarram-Gap Road fire ahead of elevated weather conditions on December 26.
"As we look toward Boxing Day, firefighters are currently conducting back-burning operations to protect nearby towns and establish fire breaks," Mr Cook said.
"Boxing Day is a big travel day right across the state, so we urge those travelling to download the VicEmergency App before you go."
Fire danger is expected to be catastrophic in the Wimmera and potentially in the Mallee while Melbourne will roast through 39C on the public holiday.
The fire has forced the Grampians National Park to close to visitors. (James Ross/AAP PHOTOS)
Similar conditions are slated for the Grampians, 270km west of the city.
Authorities said it could take weeks to fully contain the fire ignited by a lightning strike earlier in the week, with an emergency evacuation warning remaining for several communities.
Similar conditions on Saturday night resulted in the inferno moving towards Halls Gap and Lake Bellfield.
Incident controller Aaron Kennedy said the blaze was in very steep, difficult and dry terrain.
"Our ability to fight this fire directly is quite challenging," he said.
Evacuation orders are in place for towns across Victoria as firefighters battle to control blazes. (James Ross/AAP PHOTOS)
Mr Kennedy agreed the fire risk was expected to spike on Thursday, worsened by forecast windy conditions.
"This will mean that we'll see again, potentially significant fire behaviour, and then increased growth of the fire across the landscape," he said.
"Our crews are working really hard to try and contain this fire prior to the Boxing Day weather event."
Mr Kennedy said two years of dry conditions in western Victoria, particularly in the Grampians National Park, meant fires spread quickly.
The threat to the nearby communities of Lake Fyans, Pomonal, Mafeking and Watgania eased late on Saturday when emergency warnings were downgraded to watch and act, but not safe to return.
The blaze in Victoria's Grampians was moving towards Halls Gap and Lake Bellfield. (James Ross/AAP PHOTOS)
The top-level alert was renewed for Mafeking and Watgania on Sunday afternoon when changing weather conditions sent the fire northeast.
Relief centres are open in Ararat and Stawell.
The Grampians National Park, known by the Indigenous name Gariwerd, is closed to visitors.
Meanwhile, a blaze on Perth's outskirts remained out of control after threatening homes.
Residents in Two Rocks and parts of Yanchep, on Perth's northern outskirts, were warned to evacuate a few hours after the fire broke out before the warning was downgraded pending a later wind change.
Extreme fire weather conditions are forecast for several parts of NSW including the greater Sydney area.
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