Australia is sweltering, but anyone complaining about the heat should spare a thought for residents in Marble Bar where the mercury hit 48.4C at 11am.

The tiny West Australian town, which has a population of just 174 people, was officially the hottest town in the country today.

At least its long suffering residents are due for a small reprieve of sorts tomorrow when the mercury is set to dip down to 44C.

Marble Bar is used to the heat, but today it sweltered through 48.4 degrees. (Google maps)

Even the pups in Melbourne needed a swim, as the city reached 39 degrees. (AAP)

While Marble Bar is living up to its reputation as Australia’s hottest town, other parts of the country is experiencing temperatures above 40C for the fourth day in a row.

In many parts of Victoria, the mercury rose to 43C, with Melbourne itself reaching 39.

Adelaide reached 41C. Port Augusta reached 46.8.

Sydney reached a slightly cooler 27C, while Penrith in Sydney's west reached 38.

It was a day in the water in Adelaide, with temperatures reaching 41 degrees. (AAP)

An air quality forecast alert of "poor" for the Sydney region has been issued as ozone levels are forecast to exceed national air quality standards today.

Perth experienced 27 degrees, while other parts of the state, including in the Pilbara are sweltering through 46 degrees.

"Pregnant women, people aged over 65, people with pre-existing medical conditions and children and babies are particularly vulnerable on days of extreme heat," Health Victoria issued in a statement.



"Consider reaching out to someone you know that may be suffering in the heat today."

An aerial view of Green Hills Beach near Cronulla yesterday. (AAP)

Children cool off in the Nepean River at Penrith. (AAP)

According to Weatherzone, day and night temperatures will be generally eight to 12 degrees above average.

"This heat has started to build up over the north-western parts of the country and that's extended down to the south-east," Weatherzone meteorologist Jacob Cronje said.

"It's being kept in place by a high pressure system. We really won't see significant cooling over NSW and the ACT for the foreseeable future."

Western Australia declared a total fire ban in 13 districts yesterday.

A total fire ban is in place for 10 districts across South Australia including the Mount Lofty Ranges and Yorke Peninsula, as well as the Mallee and Wimmera districts in Victoria.

People living in affected areas have been urged by health authorities to drink plenty of water, avoid travelling during the hottest parts of the day and to look after those most vulnerable to the heat.

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

The latest victim is a 27-year-old man whose body was recovered after he drowned at a Victorian watering hole yesterday.

NSW Health executive director Jeremy McAnulty warned people to be prepared for the first real heat of summer.

"It's important to keep up water intake, stay cool and avoid strenuous physical activity in the heat of the day because heat places a lot of strain on the body and cause dehydration, heat exhaustion and heat stroke," he said.

Australia is bracing for a scorching end to 2018. (9news)

Dr McAnulty said ozone could irritate those with respiratory conditions and follow their action plan and take their relieving medication as necessary.

Meanwhile, as the rest of the country swelters into the New Year, the north is in for a soggy start to 2019 with thunderstorms sweeping across north Queensland with the state also on potential cyclone watch.

The sunshine state has recorded its wettest December on record.

The region has had over 400 millimetres of rain over the last two days with the wet weather expected to stick around for at least another week.

The towns of Cardwell and Ingham reported 1025mm and 902mm, respectively, for the month.

A major flood warning in place for the Daintree and Mossman rivers in Far North Queensland as the rain continues to fall.

The Bureau of Meteorology says a monsoon trough is developing north of the continent which is expected to move south in coming days.

“There is increased potential for tropical cyclone development over the next few days as well, potentially late in the weekend of early next week,” said forecaster Gordon Banks.