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

The smoke has billowed into the lower stratosphere, reaching 17.7 kilometres above sea level, 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.

Overnight, 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.