"The safety of our people is our top priority, and with hazardous smoke conditions continuing across ACT we are suspending all postie and van deliveries until further notice," Australia Post said in a statement.
A global air quality index ranked Canberra's air worse than New Delhi, India or Lahore, Pakistan for the second day in a row.
A number of national attractions, including Questacon, were forced to close on Thursday due to smoke infiltrating their buildings.
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.
"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.