The central Victorian towns of Hepburn and Hepburn Springs could be impacted by a bushfire headed directly for them.
Residents have been advised to leave the area and head toward an evacuation centre at nearby Daylesford, with concerns the fire could arrive before 10.30am.
An emergency warning has been issued for the blaze.
Firefighters battling to bring the fire under control will be bracing for hot, dry and windy conditions.
Temperatures are set to climb into the 40s in the state's central and western districts, as well as Melbourne's northwest and Geelong.
The city is set to hit 39C and Mildura 41.
A total fire ban has been declared.
CFA chief officer Steve Warrnington issued a stark warning ahead of the volatile conditions.
"The decisions you make on any given day will determine, particularly tomorrow, whether you live or die," Mr Warrington said on Saturday.
"The reality is (Sunday) is a really concerning day for us, and it is time for people to get the information they need. You must do it."
A severe fire danger is forecast for the Mallee, Wimmera, South West and Central districts.
Bureau of Meteorology forecaster Richard Russell said northerly winds in the morning will remain "quite fresh and gusty" ahead of a southwesterly change later in the day.
The change is due to hit Melbourne from 4pm, after crossing the Western Port an hour earlier, while northeastern parts of the state won't feel the relief until Monday, he added.
But temperatures may drop about 15C in less than an hour in the south of the state while thunderstorms are due to strike eastern parts without much rainfall.
It comes off the back of the warmest January on record for Victoria.
Fires have also been burning in Grantville, Timbarra, Walhalla, the Thomson Catchment, Lerderderg, Arthurs Seat, Rosebud and McCrae.
There have been 2000 bush, scrub and grass fires since the start of summer.
