Fire bans have been declared across parts of Australia, as a heatwave sweeps across the south of the country.

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

Beachgoers hoping to cool off in the sea were also advised to take care, after four people drowned in coastal waters in two days on Christmas Eve and Christmas Day.

“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.”

Victoria’s health department 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.

NSW Health has issued a warning about poor air quality in Sydney, with ozone levels expected to rise with the hot weather. Ozone exposure can irritate the lungs and people with asthma are urged to take extra precautions, such as staying inside.

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

Australians were 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.”

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 said.

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.

For Kelly Smith, the manager of Cow Bay hotel, the rain was a welcome relief after a record-breaking dry and hot spell hit the small town, next to the Daintree national park, just before Christmas.

“They’re predicting eight days of it – we’ve only had three so far but my rain gauge is already averaging 250mm every day,” she said. “Some days you can get 12 inches of rain falling in a couple of hours, our gutters can’t catch up.”

Further north at the Cooktown hotel, Matthew Fleet said the rain had cut the road south of town, stopping the bus leaving and newspapers arriving.

“Everybody has been stocking up getting ready for being locked in, sometimes we can get stuck here for a couple of weeks,” he said. “But we’re looking forward to a good crab season after this rain flushes them out, and the barra will be firing too.”

A monsoon trough was developing in the Gulf of Carpenteria. It was expected to mostly affect the Peninsula district but the entire Queensland tropics is expected to 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 was expected.