Extreme temperatures predicted for Saturday have prompted a health warning from Victoria's chief medical officer.
The Country Fire Authority is also on alert, with strong winds predicted and concerns over a spate of spontaneously combusting hay bales across the state.
The Bureau of Meteorology forecasts Melbourne will reach a top of 41, Geelong, Bendigo and Shepparton will hit 42, Ouyen and Swan Hill 43, and Mildura will be 44.
The Chief Health Officer has issued a health alert for the North Central weather district, while four other districts are near the threshold for an alert.
Northerly winds of up to 40 km/h are expected across the state on Saturday morning, sparking concerns about spontaneously combusting hay bales.
The CFA has begun preparations for the sustained period of strong winds and scorching heat, with 40 additional trucks deployed for the current fire season.
The sudden spike in temperature will be caused by a build-up of heat from central Australia, which is currently experiencing a week of temperatures in the high 30s.
The heat will be pushed from the centre of the continent to Victoria by strong northerly winds, arriving on Friday night for what will be a stifling sleep.
Victoria's Chief Health Officer Professor Charles Guest warned heat was a serious matter that people should prepare for.
"Plan ahead and take precautions to avoid heat stroke," he said.
Extreme heat can affect anybody but most at risk are the elderly, young children and people with a medical condition such as heart disease or high blood pressure.
Heat can cause illnesses including heat cramps and heat exhaustion, which can lead to heatstroke, which is fatal in up to 80 per cent of cases.
To prevent heat-related illness, people are advised to drink more water, never leave children, older people or pets in cars, check on others and stay cool.