The blaze in Victoria's Grampians National Park has razed 34,000 hectares and continues to grow despite more than 300 firefighters battling to bring it to heel ahead of extreme heat and windy conditions forecast for Thursday.
Emergency warnings are in place for those in tourist hotspot Halls Gap and surrounding areas to leave immediately.
Authorities say it could take weeks to fully contain the fire ignited by a lightning strike earlier in the week and an emergency evacuation warning remains in place for several communities.
The fire, which was sparked almost a week ago, tripled in size between Friday and Saturday due to high temperatures and strong winds.
Similar conditions on Saturday night resulted in the inferno moving towards Halls Gap and Lake Bellfield.
Incident controller Aaron Kennedy said the blaze was burning in very steep, difficult and dry terrain.
"Our ability to fight this fire directly is quite challenging," he said.
Firefighters are battling to bring a giant bushfire under control ahead of dangerous conditions. (HANDOUT/GISBORNE FIRE BRIGADE)
Mr Kennedy said the fire risk was expected to spike on Thursday when a maximum temperature of 39C and windy conditions were forecast for the area.
"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 emergency evacuation warning remained in place overnight for the Grampians blaze, with residents of several towns, including Bellfield, Halls Gap and Fyans Creek, told to leave immediately.
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.
Authorities say it could take weeks to fully contain the bushfire in the Grampians National Park. (HANDOUT/VIC EMERGENCY)
A blaze at Bullengarook, northwest of Melbourne, was also downgraded to a watch and act.
The same warning level remains in place for Bornes Hill, Grampians, Jimmy Creek, Mirranatwa, Victoria Valley, Barton, Jallukur and Londonderry.
Relief centres are open in Ararat and Stawell. The Grampians National Park is closed to visitors.
A bushfire in The Gurdies in South Gippsland was also downgraded to an advice warning.
It's been a worrying start to the bushfire season in Australia. (HANDOUT/NORTH HAMILTON FIRE BRIGADE)
Firies in NSW and Western Australia have also battled multiple bushfires in the first weeks of summer.
Temperatures in the high 30s and low 40s are expected to hit WA, including the wooded southwest of the state, over the weekend before pushing eastward.
Elevated fire dangers are forecast for the south and midwest of WA with dry thunderstorms possible for western parts.
Heatwave conditions are forecast for the state's midwest and the Kimberley and Pilbara regions.
Moderate and extreme fire weather conditions are forecast for inland parts of NSW in coming days, including the greater Sydney area.
29°C