Massive infernos have raged along the nation's east coast for months, pushing smoke across the Pacific.

Nasa said plumes from blazes around New Year's Day had crossed South America, turning skies there hazy, and moved "halfway around Earth" by 8 January.

"The smoke is expected to make at least one full circuit around the globe," the US space agency said.

Hundreds of bushfires have burnt across Australia, killing at least 28 people and destroying more than 2,000 homes.

The unprecedented scale and intensity of the fires has been exacerbated by climate change, experts say.

How has the smoke travelled around the world?
Nasa said recent blazes had been so big they had produced an "unusually large" number of pyrocumulonimbus events - or fire-generated thunderstorms.

"Once in the stratosphere, the smoke can travel thousands of miles from its source, affecting atmospheric conditions globally," Nasa said.

The agency said it was studying the effects of smoke at this altitude and whether it provided "a net atmospheric cooling or warming".

What has been the smoke's impact closer to ground?
Nasa noted the smoke had changed the colour of skies in South America, and dramatically affected New Zealand, where it was "causing severe air quality issues" and "visibly darkening mountaintop snow".

Large Australian cities including Sydney, Melbourne, Canberra, and Adelaide have also endured dangerous air quality levels due to smoke from nearby bushfires.

On Tuesday, Melbourne residents were enduring a second consecutive day of "hazardous" air quality, prompting concerns about public health.

More than 100 fires continue to burn in the nation's east, however cooler conditions in recent days and forecast rain have aided firefighting efforts.