Australian firefighters are warning about an impending "mega blaze" as efforts to tackle over 100 bushfires have backfired spectacularly, while the country prepares for its possible hottest day on record.

At least 20 houses have been razed and over 400,000 hectares burned as fire crews battle over 100 fires in New South Wales. Residents are bracing for even more extreme temperatures this coming week, as historically hot weather is forecast.

Authorities attempted to take advantage of cooler temperatures to carry out "back burning," or deliberately setting small fires to remove vegetation ahead of the main bushfires and thus stop their spread, but the tactic appears to have failed, as crews lost control of the back burn and the flames spread beyond on containment lines, reaching as high as 70m-high (229 ft).

Video footage shows a bushfire moving from treetop to treetop in a phenomenon known as ‘crowning,’ in Australia’s Blue Mountains pic.twitter.com/VSwAww0quT — Reuters (@Reuters) December 15, 2019

"In these drought conditions, whatever we try doesn't seem to be working," NSW Rural Fire Service deputy commissioner Rob Rogers said.

this is 60km away from my house. NSW is in drought and on fire.#prayersfornswpic.twitter.com/SvduQribrJ — Hannah 🐧 (@hannahrose97_) December 15, 2019

Emergency Warning for Gospers Mountain fire. Fire activity & smoke increasing. People in Wallerawang, Lidsdale & Blackmans Flat area should seek shelter as fire approaches. #nswrfs#nswfirespic.twitter.com/ueYLY6rgGw — NSW RFS (@NSWRFS) December 16, 2019

The impending heat wave could lead to the hottest day in Australia's recorded history, exceeding the current 50.7C (123.26F) record set back in 1960. So far six people have died in the bushfires since September and more than 700 homes have been destroyed, while towns and cities have been choking on noxious smoke from the wildfires. Air quality in Sydney is now worse than Beijing and New Delhi.

Bushfire smoke marking the progress of an easterly wind change over the central ranges of NSW. pic.twitter.com/XjC6wr3hB2 — Andrew Miskelly (@andrewmiskelly) December 16, 2019

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