Victorians have been told to stay inside as parts of the state brace for the hottest day in nearly two years.
Melbourne and Geelong are forecast to reach 42C on Saturday, with winds of up to 40km/h.
Bendigo, Echuca and Avalon are set to climb to 43C, while Mildura and Warracknabeal could soar to 45C.
"This heat is a killer. It's going to be like a blast furnace tomorrow and you need to adjust what you do," Ambulance Victoria's state health commander Paul Holman told reporters on Friday.
"You need to take this seriously. You need to take care. So put off the sporting events, put off the outside events, stay inside."
Saturday will be Melbourne's hottest day since January 13, 2016.
A fire ban has been issued statewide, with extreme fire warnings in the Mallee and Wimmera, and severe warnings in southwest and central parts.
A cool change in the late afternoon to early evening could cause the temperature to drop 20 degrees within hours in Melbourne.
People in regional areas have been urged to have their fire plans ready, while travellers need stay on top of the conditions in their area.
Authorities are pleading with people to act responsibly throughout the day.
"A lot of people will be returning from holidays this weekend and we ask that they consider simple steps such as ensuring campfires are completely extinguished and cigarette butts are not thrown from car windows on the way home," Assistant Commissioner Debra Abbott said.
"Use of farm machinery and tools such as angle grinders and welders should also be avoided."
Anyone found guilty of recklessly or intentionally causing a bushfire faces a penalty of up to 15 years imprisonment.
Lighting a fire on a Total Fire Ban day can also attract a fine of around $37,000 or up to two years in jail.