Melbourne will swelter through scorching top temperatures of 39 degrees on Thursday and Friday, with the mercury unlikely to dip below 25 degrees on Thursday night.

In the CBD the temperature had already reached 28 degrees by 9am on Thursday.

Temperatures in regional inland Victoria will be even hotter and staying that way longer with Mildura set to reach 42 degrees on Thursday and 44 on Friday.

Swan Hill and Horsham are 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 for Thursday and Friday.

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

"These weather conditions mean any fires that start will be much harder to control," the agency said.

Almost 140 fires burned across Victoria only a fortnight ago, when the temperature reached 41.7 degrees in Melbourne.

One home and multiple sheds were damaged by bushfire at Carrum Downs, near Frankston, on January 6.

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

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

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

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.

Agriculture Victoria has warned people to keep an eye on their animals saying "pets can suffer and even die on very hot days".

Pet owners are advised to always provide cool, clean water, ensure access to shade and never leave an animal alone in a parked car.

Relief should come for Melbourne by late Friday afternoon. A southerly cool change is set to hit about 4pm, as the temperature gradually drops by around five degrees an hour.

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