Michael Tripodi's crop of peaches at his northern Victorian orchard is late this year. And with temperatures in the region forecast to set heat records today, he's worried his fruit could boil on the branch.

Key points: Mildura in northern Victoria is expected to get to 47C today — a December record

Mildura in northern Victoria is expected to get to 47C today — a December record Much of Victoria will experience weather in the mid to high 40s

Much of Victoria will experience weather in the mid to high 40s Major building sites are expected to shut down because of the extreme heat

"They get a little gap between the stone and the flesh of the fruit," the stone fruit grower from Lake Boga, near Swan Hill, said.

"What happens in there is that it acts as a little boiler because there is a little bit of room for the sugar and moisture to boil, and it can send the fruit brown."

It would be a devastating outcome for Mr Tripodi, who needs to get his crop through today's extreme heat.

The Bureau of Meteorology expects Victoria to experience the worst of today's heatwave, with Swan Hill and Mildura in the state's north likely to reach 47 degrees Celsius.

Mildura could break Victoria's heat record for a December day.

These peaches will need to survive extreme temperatures. ( Supplied: Redland Orchards )

How hot will it get?

In Melbourne, the Bureau of Meteorology says temperatures in the CBD are expected to rise to 44C, which would be the city's hottest day since the Black Saturday bushfires.

Some of the city's suburbs are forecast to reach 45C.

Meteorologist Dean Narramore says almost all of the state will be very hot on Friday.

"Anywhere west of a line from Echuca to Melbourne is probably going to be in the mid-40s," he explained.

Melbourne is expecting its hottest day in more than a decade. ( ABC News: Margaret Paul )

Other parts of Australia will have an easier time on Friday.

Mr Narramore says a cool front should help south-western parts of Western Australia, with temperatures forecast in the high 20s.

Tasmania should also be cool, as well as coastal New South Wales, forecast to benefit from a southerly change.

But it isn't all good news.

"Pretty much everywhere inland, away from the east coast, and Victoria and South Australia, will be looking at temperatures in the mid to high 40s."

Tradies to down tools

The CFMEU says work on major Victorian building sites will shut down today because of a heat policy in the union's enterprise bargaining agreements, which sees work stop at 35C.

Jordan Paterson says workers in this week's heat need to take regular breaks to cool down. ( ABC News: Billy Draper )

Roof plumber Jordan Paterson, who has been working on smaller residential projects, will go to work, but expects to finish early.

"The heat can bounce off the roof and the walls, depending on the part of the building you are working on," he said.

Mr Paterson and his workmates apply sunscreen a few times a day, wear wide-brimmed hats, drink lots of water and take extra breaks where needed to deal with extreme heat.

"I have had heat stroke myself from this line of work and I have seen other people have to take the next day off from work due to nausea, diarrhea, headaches, so it could be really dangerous if you are not careful about it," he said.

The City of Melbourne is distributing free pool passes and movie tickets to homeless people to help them keep cool.

A total fire ban has been declared for the entire state of Victoria, as firefighters continue to battle a number of blazes, including in East Gippsland and near Colac in the state's west.

State control centre spokesman Luke Heagerty said the fire danger would be greatest in the Wimmera and South West districts, where it was rated as extreme.

Williamstown beach in Melbourne's west has been a popular spot for people to cool down during this week's hot days. ( ABC News: Billy Draper )

Ambulance Victoria is urging people not to leave children and pets in cars, after paramedics responded to 14 reports of children locked in vehicles yesterday.

Victoria's Emergency Management Commissioner Andrew Crisp said the Australian Energy Market Operator (AEMO) had advised that the state had enough electricity to get through the day, but he encouraged people to postpone using appliances such as dishwashers and washing machines where possible to ease pressure on the grid.

"I have every confidence on the advice from AEMO, there is sufficient supply," he said.