A controlled operation intended to contain a massive bushfire in eastern Australia has caused damage to up to 20 buildings, including homes, and cut off major roads, authorities say.

Firefighters had tried to use a process called backburning to tackle the Gospers Mountain blaze in New South Wales ahead of a potentially record-breaking heatwave.

Backburning, which is usually a last-resort measure, is the process of deliberately starting a small blaze in front of a main fire front.

The material that is burnt is unable to burn again, which means backburning can stop the main fire from spreading as much as it would have.

The accident occurred about 155 miles (250km) northwest of Sydney, where firefighters were trying to stop a blaze of 378,000 hectares (934,000 acres) from reaching communities.

A firefighter conducts back-burning measures in the Central Coast, New South Wales, earlier this month
Image: Backburning involves starting small fires to prevent other blazes from spreading

Rural fire service commissioner in New South Wales, Shane Fitzsimmons, told the Australian Broadcasting Corporation (ABC): “We saw a dramatic shift in conditions, a flare-up of fire, some extraordinary behaviour of that fire, and that fire has spread.”

Fire crews lost control of the backburn on Sunday afternoon. The affected properties are thought to be in the Mount Wilson and Bilpin areas in the Blue Mountains area west of Sydney.

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There were no new reports of casualties on Monday.

Bushfires have killed at least four people, destroyed more than 680 homes and burned nearly three million acres of land across eastern Australia since the start of November.

Bushfires are common in Australia’s hot, dry summers, but the ferocity and early arrival of the blazes in the spring has been unprecedented.

Experts say climate change has left bushland tinder-dry.

Spot fires that would normally be contained on their own have been fanned by wind, coming together into what authorities have called megafires to Sydney’s north, west and south.

The fires have prompted evacuations and sporadically shrouded the country’s biggest city in smoke.

Forest fire spreads 70,000 acres
November: Australia wildfires rage in New South Wales

The Bureau of Meteorology has warned of severe to extreme heatwave conditions in the inland parts of the southeast of the country, with temperatures expected to approach or exceed a national average record of 40.3C (105.4F).

Blair Trewin, bureau climatologist, said: “We saw significant heat build over Western Australia over the course of last week, and that heat is now pushing east over the continent, which is going to lead to several days of exceptional heat.

“We’re closely monitoring the development and progression of this heat but based on current forecasts we could see that record broken this week.”

The rural fire service’s Mr Fitzsimmons said the heat would exacerbate fire danger, and “despite the very best efforts of everyone, we’re not going to able to contain these fires that we’re expecting later in the week”.