Extreme heat in Australia forced the government's weather bureau to upgrade its temperature scale Tuesday, with new colours on the climate map to reflect new highs forecast next week.

Central Australia was shown with a purple area on the latest Bureau of Meteorology forecast map issued for next Monday, a new colour code suggesting temperatures will soar above 50 degrees Celsius (122 Fahrenheit).

The bureau's head of climate monitoring and prediction David Jones said the new scale, which also features a pink code for temperatures from 52 to 54 degrees, reflected the potential for old heat records to be smashed.

"The scale has just been increased today and I would anticipate it is because the forecast coming from the bureau's model is showing temperatures in excess of 50 degrees," Jones told Fairfax newspapers.

Australia's all-time record temperature is 50.7 degrees, set in January 1960 at Oodnadatta in the state of South Australia.

The nation as a whole experienced its hottest day on record on Monday with the average maximum temperature across the country hitting 40.33 degrees, surpassing the previous mark of 40.17 degrees set in 1972.

That record was likely to have tumbled on Tuesday, Jones said, with scorching temperatures across much of southeastern Australia including in Sydney where the mercury topped 42 degrees.

More than 100 fires blazed in New South Wales, Australia's most populous state, where officials described the heat, wind and dry vegetation conditions as among the worst the state had ever seen for wildfires.

"We had the hottest day on record for Australia (on Monday) and today it looks like we may well go better again," said Jones.

"This really puts the national dimension of this heat event into bigger context."

Aaron Coutts-Smith, the weather bureau's climate services manager, said the heat's extent was unprecedented.

"What makes this event quite exceptional is how widespread and intense it's been," he said.

"We have been breaking records across all states and territories in Australia over the course of the event so far."

Speaking about devastating wildfires that razed 100 homes in the southern island state of Tasmania at the weekend, Prime Minister Julia Gillard warned this week that Australia would see more extreme events due to global warming.

"Whilst you would not put any one event down to climate change, weather doesn't work like that, we do know over time that as a result of climate change we are going to see more extreme weather events and conditions," Gillard said Monday.