A 10-kilometre stretch of the Hume Freeway in central Victoria has melted in the heat, with the state's police advising motorists heading towards Melbourne to prepare for delays.

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It comes as parts of the country prepare for heatwaves today and tomorrow, with the mercury set to soar above 40 degrees in areas of New South Wales, Victoria, Queensland, Western Australia, the Northern Territory and South Australia.

A spokeswoman for VicRoads said hot weather could result in "bleeding of the road surface, which occurs when the bitumen becomes reactivated by warm temperatures and becomes soft and sticky".

"This, combined with higher than usual traffic volumes, has affected the pavement on the southbound carriageway of the Hume Freeway between Tallarook and Broadford," VicRoads said.

The temperature in the area hit just over 30 degrees Celsius at 5:00pm on Friday afternoon.

VicRoads said the site would be monitored after repairs were carried out last night.

"The contractors are currently working to reopen both lanes to clear the traffic backlog, however one lane southbound will remain closed during the repairs tonight and tomorrow," VicRoads said.

Cars backed up on the Hume Freeway near Seymour. ( Facebook: John Lang )

"Motorists are advised to expect delays or take alternative routes if possible."

Prepare for hot weekend

The issue comes as authorities are warning Victorians to prepare for a hot, dry and windy day today, with temperatures set to top 40C for much of the state.

Mildura is expected to reach 44C.

The weather bureau said a gusty southerly change is expected to sweep across the state, reaching Melbourne in the late afternoon or early evening today, bringing relief from the heatwave conditions.

Senior forecaster Michael Logan said the temperature was expected to plummet by up to 20C when the change arrives in the Melbourne area.

"The heat will really build all day across Victoria tomorrow [Saturday] and for many places the heat will probably peak just before that squally change comes through in the afternoon and evening period," he said.