Firefighters tackling bush fires Australia in were hampered when the wind changed direction, pushing the flames back towards them.

Soaring temperatures in the country's densely-populated southeast helped fan the flames of blazes near Sydney, Melbourne, Tasmania and Victoria.

Fires in the Tasmanian world heritage area threatened six towns and sent thick smoke across Hobart and Port Arthur and by Saturday morning a blaze had ripped through 15,000 hectares (37,000 acres) of southwestern wilderness.

The sky over Tasmania's Constitution Docks turned a brown-reddish colour, prompting people to post images of it on social media.

One fire in eastern Victoria, the country's second most populous state, prompted fire authorities to issue a watch and act warning for residents in 14 different towns.

The fire near Rosedale, about 124 miles east of Melbourne, had burned more than 10,000 hectares (25,000 acres).

Sudden wind changes on Friday created risky conditions for roughly 40 firefighters, the state's emergency management commissioner said.

“They found themselves in a very serious situation and they were shaken,” Andrew Crisp added. “I have spoken to both the chiefs of those organisations, but they’re safe and well and they are being well looked after and we need to and we will learn from this.”

Mr Crisp added that a fire Rosedale fire and another in Victoria were being treated as suspicious, with police and fire authorities investigating their cause.

Victoria and Tasmania had sweltered through above average temperatures on Saturday. Earlier this week Melbourne recorded a near-record 42C (107.6 F) and Hobart hit 40C (104 F).

A rapid cool change in weather will see far cooler temperatures sweep both states this weekend.

“At 4.00 p.m., (Melbourne) was at 42 degrees, by 4.20 p.m. it was down to 26 degrees, and then continued to drop after that,” said Keris Arndt, a meteorologist with the weather bureau.

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While it was hoped that falling temperatures would also bring relief for Sydney, by Saturday morning the cooling whether had not reached Australia's most populous city, where the mercury reached almost 38 C (100.4 F) by midday.