Melbourne is expected to hit 42C in a second sweltering day before a cool change, but the north of the state will feel the heat well into the weekend.
People across the state sweated through a hot night with the midnight temperature hovering at 30C in Melbourne, before eventually dropping to the low 20s at about 4am.
But after 9am the heat had again marched past 30C in the CBD.
"Melbourne's looking at a maximum of 42C this afternoon occurring about 3pm, but it will be followed by a cool change very shortly in the CBD, between 3pm and 4pm," Bureau of Meteorology senior forecaster Michael Efron told AAP.
"That will drop temperatures by about 10 degrees in 10 to 15 minutes."
However, Melbourne's outer northern and eastern suburbs will not get as sharp a temperature drop, Mr Efron warned, and the north of the state will keep sweating into the weekend.
Mildura and Swan Hill are forecast to reach about 45C, and in the east of Victoria Shepparton and Gippsland will see temperatures between 40C and 42C on Friday.
But a cool change has already swept through the southwest of Victoria, with Portland and Warrnambool expecting temperatures in the mid-to-high 20s, Mr Efron said.
Emergency Services Commissioner Craig Lapsley has warned people to keep safe during the heat.
"Yesterday was hot and last night uncomfortable. We had fires and we'll have more today," he tweeted on Friday.
"We'll see heat impacts on people and infrastructure. It's our summer but we need people to be tuned to where they are, to take care and to check in on others."
Temperatures peaked at 39C in Melbourne on Thursday, but some parts of the state were already in the early 40s.
Ambulance Victoria responded to 15 heat related calls on Thursday, taking six patients to hospital, and nine reports of children in cars. None of the children needed hospital treatment.
Firefighters are bracing themselves for more blazes, after spending Thursday tackling two large fires in regional Victoria.
A fire at Mumbannar, on the South Australian border, is contained but not yet under control.
Meanwhile, a grassfire at Dreeite South, near Colac in the state's southwest, is under control after destroying two derelict houses and five sheds, and killing dozens of sheep.
Trains on the regional and metropolitan networks saw massive delays because of signal faults and track expansion in the heat.
V/Line has warned speed restrictions will apply again on Friday, and the regional lines also have coach replacements due to planned maintenance.
