Washington, Dec 23 (IANS): Millions of residents and visitors in southeast Australia have been warned to brace for catastrophic bushfire conditions over the Christmas period.

Authorities on Monday declared a total fire ban for greater Sydney and northern regions of the state of New South Wales (NSW) due to the threat posed by high temperatures and strong winds, reports Xinhua news agency.

It comes ahead of a period of forecast extreme danger across Australia's southeast.

The Bureau of Meteorology (BoM) has forecasted that temperatures in Sydney, Melbourne in the southeastern state of Victoria and Adelaide in South Australia will exceed 35 degrees Celsius on Wednesday and Saturday.

In Victoria and South Australia, authorities have warned that conditions on Thursday will pose a catastrophic bushfire threat, with strong winds expected to make it more challenging to contain fires.

Dozens of bushfires are already burning across the three states, which together account for 62 per cent of the Australian population, including a fire in the Grampians National Park in western Victoria that has triggered evacuation warnings for several towns.

As of Monday, the fire has burnt through 41,000 hectares of land in the seven days since it was ignited by lightning.

Emergency warnings for communities in and around the national park were downgraded on Monday due to favourable weather conditions and light rain, but authorities have warned the situation could change at any time.

State Response Controller Garry Cook said on Monday that firefighters were conducting controlled backburning operations ahead of Boxing Day to protect nearby towns and reduce the fuel load in the area.

Firefighting crews across Australia's east coast have been deployed to aid local personnel.

Anyone travelling through Victoria over the Christmas period has been advised to keep up to date with local emergency warnings and monitor conditions.