According to the World Health Organisation's air quality index, Gunnedah (261) and Tamworth (251) had the second and third worst air quality in Australia, narrowly behind Bathurst (262).

The smoke blanketing the area was blown in from the ongoing bushfires raging across the state, and put the region on the global map for the wrong reasons.

The air quality of the New England and the North West was much worse than infamously polluted cities such as Beijing (225), Mumbai (185), Shanghai (180) and Jakarta (156).

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The number of PM2.5 particles was more than four times above the most severe 'hazardous' rating, prompting the NSW Department of Planning, Industry and Environment (DPIE) put out a health warning.

"People with heart or lung disease, older adults and children should avoid exercising outdoors," the DPIE alert states.

"Everyone else should avoid prolonged or heavy exertion."

Tamworth RFS operations officer Matt McKean said the smoke also posed a problem to the region's firefighters, with difficult conditions predicted for the rest of the week.

"It makes it difficult to pinpoint fire locations, as there is no individual column of smoke," Mr McKean said.

"It also makes it harder for us to activate our air resources. If visibility is low, we can't launch aircraft out of the air base."

Firefighters will be on high alert, due to an unfavourable combination of "elevated temperatures, low humidity and potentially escalating winds".

"We're expecting a very high fire danger rating to persist over the coming week," Mr McKean said.

"We're not expecting much relief, and we're asking the community to remain vigilant over Christmas."

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