Overnight on Monday, the temperatures were warm, dropping to the minimum of 20.5 degrees at about 1am.

Bureau forecaster Stephen King said Tuesday's forecast was revised down to 35 degrees, as a southerly change will come through much earlier in the day.

"There is a little bit more cloudy cover that we're anticipating and the southerly change will be earlier than we thought it would have been," Mr King said.

While hesitant to put a time estimate on when the cool change will hit, he said temperatures in Melbourne would ease after lunchtime.

"The southerly change will probably happen in the early afternoon, it will come up from the south and creep north across the city during the afternoon," Mr King said.

By 11.30am forecaster Dean Stewart said the southerly winds were developing over Port Phillip Bay, which would spread over the city and keep the temperatures in the mid to low 30s for most of the afternoon.

"It won't be a marked cool change but it will prevent the temperature from rising too high, so hopefully it will come through the city soon," Mr Stewart said.

"Towards this evening it will gradually drop away but it will stay in the 30s throughout the afternoon."

He said the highest temperature was at Essendon Airport, which had recorded 37 degrees, Melbourne CBD had reached 34.9 degrees just before midday.

The change will be welcome news for tennis players and spectators on day two of the Australian Open.

In 2014 and 2009 organisers enacted the event's heat policy to suspend play.

Play is only called off once the "wet bulb globe" temperature – based on the advice of the weather bureau, a meteorologist and the tournament doctor – reaches a level of both heat and humidity that organisers believe is too high.

The state's north is still set to swelter through 40-degree-plus temperatures, with a top of 43 degrees forecast for Swan Hill, 42 for Mildura and 41 for Echuca.

Country fire fighters are preparing for the heat, a total fire ban was declared on Monday afternoon for the Mallee, Wimmera and North Central districts.

The CFA said conditions are the hottest and driest so far this fire danger season.

"We've had a spate of grass fires in a number of areas, which have been challenging for firefighters given the high amount of grass and scrub growth," CFA Chief Officer Steve Warrington said.

"Northern areas like Mildura have been experiencing hot and dry conditions, which increases the risk of fire.

"[On Tuesday] across the state, there is potential for fast-moving grass and forest fires.

"Know what you can and can't do on a day of Total Fire Ban and ensure you stay informed about conditions; make sure you look after yourself in the heat and check in on your neighbours."

Much of the grassland in the state is now completely dried out, with the risk of grassfires extremely high, CFA state duty officer Gary Weir said.

From Wednesday, the temperature in most of Victoria will drop by 10 degrees, and Melbourne will see showers and wintry conditions in the early 20s until Sunday, when the mercury will jump back up to 30 degrees.

For more on what you can and can't do during Total Fire Ban days, when fires in the open air are legally restricted, visit the CFA website.