Melbourne is forecast to reach a scorching 38C on Friday as northwest parts of the state surpass 40C.
The temperatures have prompted a total fire ban and warning from health authorities for some areas.
"The chief health officer has issued a heat health alert to notify you of forecast extreme heat conditions at or above heat health temperature thresholds," the government said in a statement.
The warning covers areas including west and south Gippsland and the Wimmera.
The Country Fire Authority has also imposed a total fire ban for the districts of the Mallee, Wimmera and the Northern Country.
The Bureau of Meteorology said the temperatures mean Victoria will record its hottest start to summer in 24 years.
"There is a change coming, every heatwave has to end eventually, however this is not a very strong change," senior forecaster Chris Godfred said.
"We should see it reach Melbourne very late on Friday night so unfortunately we're not going to get real relief from the heat until Saturday."
Australian Associated Press