It was still 26 degrees at 5.30 on Friday morning and it was not until after 1am that the temperature dipped below 30.

Firefighters on Thursday battled two fires in western and south-western Victoria, with residents earlier advised to flee amid soaring temperatures.

One thousand homes were left without power after an outage in Beaufort, west of Ballarat, believed to have been caused by a car crash.

And peak-hour commuters experienced severe delays on the V/Line network and minor delays across the Metro network.

Soaring temperatures

On Thursday, the searing heat saw top tennis players struggle to play in temperatures of up to 69 degrees on centre court.

The mercury hit 40 degrees in Melbourne at 5.30pm, and the overnight temperature is unlikely to dip below 25 degrees before climbing to 30 degrees by 9am Friday.

Across the state, temperatures hit 42.9 degrees in Dartmoor, 42.8 in Hopetoun, 42.6 in Walpeup and 42.3 in Warracknabeal.

And if the expected top of 42 degrees is reached on Friday, it will be the hottest day in two years.

Rising forecasts

The sudden surge in Friday's forecast is due to a later-than-expected cool change, Bureau of Metrology senior forecaster Dean Stewart said.

"The cool change expected is a touch later than we originally anticipated," Mr Stewart said.

"Instead of coming through at 2pm, we now believe it will come through the city closer to 4pm, which means there is extra time for the mercury to rise higher and temperatures to increase."

Mr Stewart warned the city to brace for "pretty stifling conditions" as the extreme heat bears down on the state.

"It's going to heat up very quickly and there will be sunny conditions with strong northerly winds, which will make it extremely hot and uncomfortable," he said.

By midday the city will be sweltering, with the temperature predicted to rise to 37 degrees before it hits the maximum temperature of 42 degrees between 2 and 4pm.

It is expected to top the 41.7 degrees reached on January 6 this year, making it the hottest day since January 13 in 2016.

A southerly cool change is set to hit after 4pm, and the temperature should gradually drop by around five degrees an hour to an overnight low of 19 degrees.

"It will be a welcome relief for a lot of people but it looks like it will be a dry cool change, which isn't going to produce any showers," Mr Stewart said.

He said the cool change was expected to hit Geelong early afternoon before it rolls into Melbourne. Other parts of the state will not be so lucky, with the heat persisting throughout Friday night.

Health alerts

The Department of Health and Human Services issued public health alerts for Thursday and Friday.

The department said older people, young children and people with medical conditions were most at risk, but the extreme heat could affect anybody.

People are being encouraged to drink water, be aware of local conditions and look after themselves and others.

Regional heat

The bureau predicts temperatures in regional inland Victoria will be even hotter, with Mildura set to hit 44 degrees on Friday.

Swan Hill and Horsham are also predicted to reach 44 degrees on Friday and temperatures throughout the region will hover in the low 40s over the weekend.

The Country Fire Authority has issued total fire bans for the Mallee, Wimmera and South West regions and fire danger is rated very high in North Central, Central and North East districts.

Residents have been advised to activate their bushfire plans on Friday.

Parks Victoria also issued a warning about lighting campfires or barbecues in fire ban areas.

Australian Open

The heat may even interrupt play at the Australian Open.

Tennis Australia's heat policy indicates that matches may be suspended - at the discretion of the umpire - if ambient temperatures reach 40 degrees.

Tennis Australia's chief operations officer Tom Larner said matches were unlikely to be halted based on the forecast, but that plans could change with conditions.

"The players prepare for this, they come early to acclimatise to our weather, they know Melbourne can be 18 degrees or 38 degrees," he said.

Saturday is forecast to reach 26 degrees with temperatures to hover in the mid-20s for the next week.

Parks to close

Some parks across the state will be closed on Friday due to the fire danger. They include:

Dandenong Ranges National Park (including 1000 Steps); Dandenong Ranges Botanic Garden (formerly National Rhododendron Gardens); William Ricketts Sanctuary; Alfred Nicholas Memorial Gardens; George Tindale Memorial Garden; Organ Pipes National Park; Pirianda Garden; Plenty Gorge Park - Yellow Gum Picnic Area (including Blue Lake); RJ Hamer Arboretum; Silvan Reservoir Park; Sassafrass Creek NCR; Woodlands Historic Park; Warrandyte State Park visitor areas: Jumping Creek Reserve, Koornong Reserve and Norman Reserve; Bellbird picninc Area at Yarra Bend Park