Victoria is experiencing its hottest start to summer in almost a quarter of a century, as authorities maintain health alerts for parts of the state.

Melbourne is due to reach 35C on Thursday while some centres in the state’s northwest could surpass 40C according to the Bureau of Meteorology.

“Heat has been building, fanned by a northerly breeze … there’s plenty of sunshine, no real cloud,” bureau senior forecaster Chris Godfred said.

The state government has warned that the hot conditions pose a health risk for people in districts including the Mallee and Wimmera.

“The chief health officer has issued a heat health alert to notify you of forecast extreme heat conditions at or above heat health temperature thresholds,” a statement read.

The hazard will continue on Friday when temperatures are expected to increase further, with Melbourne stretching to 38C and inland regional centres including Mildura due to record 40C.

“We haven’t seen temperatures this high this early in December since 1994,” Mr Godfred said.

The fire danger on Friday will be high.

“There is a change coming, every heatwave has to end eventually, however this is not a very strong change.

“We should see it reach Melbourne very late on Friday night so unfortunately we’re not going to get real relief from the heat until Saturday.”

The weekend is forecast to be unsettled, humid with some showers across the south of the state, while some northeast areas might get hotter temperatures than Friday.

© AAP 2018