The new record was set at 4pm this afternoon, 1.6 degrees above the previous record set on New Year's Eve 2005.

UPDATE, 2.30pm: Bendigo has officially broken its December heat record.

The mercury hit 43.3 degrees at 2.20pm, according to the Bureau of Meteorology.

The previous high was 43.1 degrees, recorded in 2005.

The temperature is expected to climb to 45 degrees this afternoon.

Update, 2pm: An air quality warning has been issued on Friday for all of Victoria.

An Environment Protection Authority spokesperson said people would see or smell smoke because of the fires in New South Wales and Gippsland.

The smoke would continue to impact air quality on Saturday morning, the spokesperson said.

Smoke and dust may reduce driving visibility, so people should drive to conditions.

Anyone experiencing wheezing, chest tightness, and difficulty breathing should call 000.

Those who are sensitive to smoke should limit prolonged or heavy physical activity. Where possible, people should try to stay indoors.

UPDATE, 12 noon: Bureau of Meteorology senior meteorologist Keris Arndt says a cool change is on track for this evening as temperatures continue to climb through the day.

Mildura hit 40 degrees at about 10.30am.

"It's tracking just as we thought it would. It's warming up very quickly after hot conditions in west of the state," Mr Arndt said.

"We are expecting temperatures to keep climbing as day progresses. Melbourne will hit 40 degrees by about 2pm and stay up around there until 8pm or 9pm.

"It's going to stay very hot in the afternoon with hottest period between 4pm and 5pm.

"But the cool change is on track, just a bit later than people might want. It will hits the south west of the state about 5pm and reach Melbourne around 10pm, so it's still going to be a pretty warm night.

"The northeast will still be warm tomorrow morning but is set to cool down after that."

The chance of dry thunderstorms is also possible for parts of the state.

"As the change comes through we have the chance of dry thunderstorms and very little chance of rainfall," Mr Arndt said. "So we might get gusty conditions which can be pretty bad."

UPDATE, 8am: Ambulance Victoria is urging people to not leave children and pets in cars after paramedics were called to 14 reports of children locked in vehicles on Thursday - including one in the Loddon Mallee region.

A spokesperson said no one required treatment.

UPDATE, 6.40am: The Bureau of Meteorology has issued fire weather warnings Mallee, Wimmera, Northern Country, North Central, South West and Central forecast districts.

The CFA advises people to action their bushfire survival plan, monitor the fire and weather situations and call Triple-0 in an emergency.

The weather and fire warnings are expected to remain in place all day today.

With the mercury has already climbing 22 in Bendigo (as of 6.30am), severe fire danger is expected in the Bendigo area.

Forecasts for Bendigo expect a late change today with winds turning northwesterly in the early afternoon.

Saturday is expected to reach a top of 30 in Bendigo with a small chance of showers in the morning.

A top of 36 is forecast for Christmas Day.

EARLIER: Bendigo will likely bake through its hottest December day on record today.

The mercury could reach 45 degrees, eclipsing a previous record of 43.1 degrees on New Year's Eve, 2005, Bureau of Meteorology forecaster Steven McGibbony said.

"We've had a lot of hot air building up over the interior of the continent. We expect a cold front to approach, pushing winds tending northerly and that will drag all that air across to Victoria," he said.

If reached, the 45 degree temperature peak would be close to Bendigo's all time high of 45.9, which was reached last February.

The BOM's Andrew Tupper said it was not the record that authorities wanted to highlight, but rather that the public needed to be alert to health and fire risks.

"The reason is that we want to get the message out that this is not a usual summer," he said.

A severe fire risk is in place across central Victoria, with gusty conditions expected to hit very late in the day as the cool change rolls in.

Bendigo Health is preparing in case it sees more people coming in with heat-related issues, acting emergency department director Simon Smith said.

"Certainly we've gone out and we've got equipment that can deal with treating these patients if necessary. We have plans in place to deal with that," he said.

"A lot of it is common sense. Try to restrict unnecessary travel, keep well hydrated and keep in a cool space.

"It's something that everyone knows what to do but perhaps just need to be a little bit more aware on days like tomorrow. They need to take it a little bit more seriously."

Paramedics are preparing for a hot day but the heatwave has already kept them busy, particularly on Wednesday in Bendigo and Mildura, Ambulance Victoria's Justin Dunlop said.

They asked people to plan to stay out of the heat, keep water on them and not leave children in cars.

"So far today we've had six," Mr Dunlop said.

The temperature should drop to the low 30s over the weekend, but Christmas day could see 36 degrees.

Mr McGibbony said it remains unclear whether the heat seen this week was a harbinger of more record breaking weather to come over the rest of summer.

"It's a bit hard to talk specifically about the temperatures we can expect," he said.

"But we are in the early part of summer and there are plenty more opportunities for hot weather and heatwaves."