The cool change and cloud cover will see maximum temperatures drop to the mid-20s across southern parts of the state on Saturday, some 20 degrees cooler than the north and west of the state.

Bureau of Meteorology senior forecaster Michael Efron said Melbourne would hit a top of 26 degrees on Saturday with the chance of showers in the afternoon and possible thunderstorms overnight.

Sunday in Melbourne would reach 29 degrees with the risk of more storms in the early morning and then again in the afternoon, he said. Monday should be a pleasant 25 degrees with cool southerly winds and patches of sunshine. The next two days are both set to be relatively mild as well, with a top of 23 predicted on Tuesday and Wednesday likely to reach the mid-20s.

???And then the mercury is set to rise.

"On Thursday the winds swing round to the north, with Melbourne going back up to 31 degrees and Mildura reaching 40 degrees," Mr Efron said.

"It looks like it'll be even warmer on Friday - 35 in Melbourne and 42 in Mildura.

"At this early stage, Saturday looks set to be very hot across the majority of the state.

"Melbourne will be nudging 40, maybe 39 degrees and, across the north, low to mid 40s but remaining humid as well."

Saturday's cool change in the south also stands in stark contrast to conditions across the west and north of the state, with the mercury forecast to top 45 degrees in Mildura and Swan Hill on Saturday and to remain high throughout most of the week.

Emergency Victoria has issued an extreme heat in Mallee, North Central, North East and Northern Country forecast districts for Saturday.

V/Line passengers are being warned of delays of up to 30 minutes and some coach services replacing trains due to heat speed restrictions.

Bushfires are burning in the Big Desert and Wyperfeld National Park, sparked by lightning on Friday evening.

Mr Efron said storm activity throughout the week would likely exacerbate the fire danger.

"It's not expected to bring significant amounts of rain, if anything, it could be more of hazardous in terms of dry lightening and also it can generate quite erratic and gusty winds as well," he said.