Canberra's air quality is the worst out of any major city in the world due to smoke from bushfires along the NSW South Coast.

Conditions have even affected some MRI machines at Canberra Hospital, but a health spokesman said the hospital was still meeting demand.

Australia Post has also suspended mail deliveries in Canberra until further notice, saying staff safety was a priority.

Parliament House is seen through thick smoke haze in Canberra on New Year's Day in 2020. The 2pm air quality index reading at Monash was 4650 - more than 23 times the hazardous level of 200. (AAP)

A global air quality index ranked Canberra's air worse than New Delhi, India or Lahore, Pakistan.

But conditions had been worse in the NSW coastal town of Bateman's Bay, which has been besieged by fires in recent days, rating nearly twice as bad as Canberra earlier on Wednesday.

Air quality in Canberra is more than 10 times hazardous levels according to local health authorities, having peaked at more than 20 times hazardous levels on Wednesday.

The smoke is expected to linger over the capital in the coming days, with the Bureau of Meteorology saying high temperatures were trapping the smog there.

There are out of control fires burning near Kosciuszko National Park to Canberra's west, and the fires along the coast to the east.

Bureau forecaster Abrar Shabren said this meant whatever the wind change, smoke would still be blown across the territory.

Thick smoke haze seen over Canberra Airport. (Dion Georgopoulos)

"It will persist over the next couple of days. We will see a change probably tomorrow," he said.

Hazardous levels are normally considered 200 or above, with air quality stations around Canberra recording levels between 2300 to 2800.

ACT Health has urged Canberrans to stay indoors and avoid physical activity outdoors.