Smoke and haze hangs over mountains near Cooma, New South Wales. Image taken from a P-8A Poseidon conducting damage assessment and surveillance in the bushfire affected area.

The smoke from the Australian bushfires is so severe it is expected to complete a circuit of Earth, returning to the country's skies from the west.

The smoke has billowed into the lower stratosphere, reaching 17.7km above sea level, the US space agency Nasa said this week.

It comes as major Australian cities are still struggling with low air quality from bushfire smoke. Wangaratta on Tuesday morning was the third worst city in the world for air quality, bumping the Melbourne suburb of Doncaster from fifth to sixth place. The Indian city of Singrauli was the worst, according to the World Air Quality Index project.

On Monday night, Melbourne's air quality was the the poorest across the globe, Victoria's Chief Health Officer Brett Sutton said. He expected it would improve during the day on Tuesday.

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VICTORIAN GOVERNMENT Australia is a bushfire-prone nation. But several factors make this fire season worse than those past.

"The smoke is expected to make at least one full circuit around the globe, returning once again to the skies over Australia," Nasa stated.

"Over the past week, Nasa satellites have observed an extraordinary amount of smoke injected into the atmosphere from the Australian fires and its subsequent eastward dispersal."

Mark Baker/ AP Bushfire smoke shrouds the skyline of Australia's largest city Sydney on Jan 4.

Bushfires have already burnt more than 5.2 million hectares of land in New South Wales and 1.3 million hectares of land in Victoria this fire season.

The smoke is having a dramatic impact on nearby New Zealand, which has experienced severe air quality issues and a darkening of the colour of the snow on the mountains.

Nasa satellites show smoke has travelled more than 6500km away from Australia, with some of it reaching Chile, where hazy skies and colourful sunsets have been reported.

The space agency has also labelled the Australia's bushfire-generated storms, or pyrocumulonimbus events which have been exacerbating fire activity this week, as "rare".

Fiona Goodall/ Getty Images An orange glow darkens the sky in Aucklan, New Zealand after the smoke travelled over from Australia.

Mike Fromm of Nasa's Naval Research Laboratory said that by the agency's measures, it was "the most extreme pyrocumulonimbus storm outbreak in Australia".

A pyrocumulonimbus occurs when moisture trapped in the smoke condensed in the cold upper air produces a cloud, which then produces its own lightning.

"Large and numerous pyrocumulonimbus events are relatively rare - especially at this scale," Chip Trepte, a project scientist from the research body CALIPSO at Nasa's Langley Research Center said.

The smoke has been tracked by satellite data which is used to create an ultraviolet aerosol index.

The UV index is particularly well suited to tracking smoke from pyrocumulonimbus events, according to Nasa Goddard' research scientist Colin Seftor, as the higher the smoke plume, the larger the aerosol index value.

"The aerosol index values produced by some of the Australian pyrocumulonimbus events have rivalled the largest values ever recorded," he said.