Extreme record-breaking heatwave conditions are forecast to sweep across four states over coming days, sparking health and fire warnings.

A broad area stretching across much of southern Australia is set to experience the hot weather, with temperatures generally 10C to 14C higher than usual for this time of year, the Bureau of Meteorology said.

The mercury is expected to soar above 40C in many parts, and residents in Mildura, in north-west Victoria, could see maximum temperatures stay above 40C for four days in a row for the first time in 90 years.

“Summer is well and truly here with warm to hot conditions developing across southern Australia to finish the year of 2018,” a meteorologist, Jonathan How, told reporters on Wednesday.

“We are seeing severe to even extreme heatwave conditions develop across quite a wide area of southern Australia, stretching from Western Australia in the Pilbara district, through South Australia into southern New South Wales and Victoria as well.”

South Australia and Victoria will face an elevated fire risk, while Victoria’s health department has issued a warning the conditions may lead to increased heat-related illnesses.

“Extreme heat can affect anyone. Pregnant women, people with a pre-existing medical condition, people aged over 65 and children and babies are most at risk,” the department said.

How warned the hot conditions could be here to stay, with the heat sticking in Canberra and Sydney right up until the new year. “The heat isn’t unusual for this time of year, but what is unusual is how protracted and extended the heat is,” he said.

Canberra could record temperatures above 35C for five consecutive days, and western Sydney can expect an extended period of above 40C.

Marble Bar in Western Australia is set to hit 49C on Wednesday and might see a run of eight days above 45C for the second time this month.

Temperatures are expected to hit 35C in Melbourne and 41C in Adelaide in the coming days, but should cool at the weekend.

Australians have been warned to expect similar extreme conditions over the coming months.

“This is the first month of summer, so there is a long way to go for much of the country,” How said. “At the moment the bureau is forecasting a warmer and drier summer than normal, so this could really just be a taste of things to come.”

But in north Queensland the wet season has begun in earnest, with heavy rains and thunderstorms likely to increase in coming days. A flood watch was issued for areas between Rollingstone, just north of Townsville, and Cooktown, the Bureau of Meteorology says.

Falls of up to 218mm were recorded near Cairns and up to 180mm had fallen near Cooktown over the past 24 hours, a meteorologist, Adipi Sharan, said on Wednesday.

A monsoon trough was developing in the Gulf of Carpenteria, she said. It would mostly affect the Peninsula district but the entire Queensland tropics would be wet right into the new year.

In the state’s south-west, Birdsville was forecast to peak at 42C on Wednesday. The high temperatures would carry through until New Year’s Day, when 44C is expected.