A heatwave will hit most of Australia this week that will push Christmas temperatures in some areas to as much as 12C above the December average before breaking monthly records later in the week.

Victorian authorities have issued health alerts for parts of the state and fire warnings are in place for several other states including Western Australia, South Australia and Tasmania.

The Kimberley region of Western Australia is the one location that could break heat records on Christmas Day, with the town of Fitzroy Crossing expecting a maximum of 47C, breaking the previous annual record of 46.5C, which was set earlier this month.

Inland areas of South Australia will have temperatures in the low-to-mid 40s and Adelaide is forecast to reach 41C on Thursday.

Parts of Victoria are forecast to reach temperatures above 40C towards the end of the week, with Mildura in the state’s north-west expecting a top of 46C on Sunday.

The heatwave is expected to intensify across a large swathe of Australia later this week. Photograph: Bureau of Meteorology

Other parts of the country could see record highs for December later this week.

“Generally, for Christmas Day across southern Australia we’re looking at temperatures 6C to 12C above average for the daily maximum,” said Sarah Fitton, an extreme weather meteorologist at the Bureau of Meteorology.

“It’s a little bit cooler for Tasmania and southern Victoria for Christmas Day, but still above average for this time of year.”

Coastal Queensland is the only region experiencing below-average temperatures for this time of year.

Temperatures across southern and central Australia and New South Wales will increase from Boxing Day and into the new year period.

The Victorian health department has issued a health alert for the Mallee, north central, north-east and northern country (which takes in towns such as Bendigo and Shepparton) and Wimmera districts effective from Thursday.

“The forecast conditions may cause an increase in heat-related illnesses in the community,” the department said and warned people older than 65, children and babies, pregnant women and people with medical conditions were most at risk.

On Monday afternoon the SA Ambulance Service issued a similar warning for the Christmas period and said they had extra doctors, paramedics, nurses and call centre staff working because of the forecast conditions.

In Sydney, temperatures are forecast to reach 34C in city areas on Sunday, 38C in Parramatta and 41C in Penrith and Richmond.

Areas around Adelaide and Melbourne will have milder temperatures around Boxing Day before the heat kicks in again.

Fitton said the heatwave was the result of hot air building up to west of the country where there hadn’t much monsoonal activity that would usually be expected this time of year.

“The heat’s building up and the skies are clear so there’s no cold fronts coming through,” she said.