SYDNEY - Millions of people in southeast Australia have been warned to prepare for severe heat and possible fires.

Temperatures across Victoria, Australia's second-most populous state, and in parts of South Australia were forecast to surpass 40 degrees Celsius on Monday, prompting widespread emergency warnings.

In Melbourne, the capital of Victoria, the temperature is forecast to hit 41 degrees Celsius, the city's hottest December day since 2019.

A total fire ban has been declared for most of Victoria and firefighting authorities across the state have been put on high alert.

Meteorologists have warned that dry lightning storms could elevate the fire risk.

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Jason Heffernan, chief executive of the Country Fire Authority, said at a press conference on Sunday evening that a combination of the heat and dry winds would make for horrendous fire conditions in Victoria's west.

"If fires do start in the landscape, they will be hard to suppress, they will spread very quickly," he said.

A Boeing 737 Fireliner with the capacity to carry and dump 15,000 liters of water or fire retardant has been deployed to the state.

Chris Hardman, chief fire officer from Forest Fire Management Victoria, said that 54 firefighting aircraft have been moved around the state to ensure they are located close to where the risk is.

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The state government has issued a heat health warning, advising people to stay indoors and urging them to check on those most at risk, including the elderly.