Local health authorities said levels peaked at more than 20 times hazardous levels on Wednesday, with smoke expected to linger in the coming days.
As high temperatures trap smog in the air, Australia Post took the decision to cancel deliveries from Thursday.
“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,” a statement read.
“We encourage customers with inquiries about their mail to check the Tracker Tool or contact us on 13 POST.
“We apologise for any inconvenience, however, we must do what is right for our people. We thank customers for their patience and understanding.”
Hazardous levels are normally considered to be at an air quality index of 200 or above.
Air quality stations around Canberra recording levels between 2300 to 2800.
Deliveries in a number of areas across Victoria, NSW and Western Australia have also been affected by bushfires, while 10 post offices have been closed.
“Safety is our main priority, and we are reaching out to our people and supporting them through this tough time,” Australia Post said.
“We are monitoring the situation closely, and will provide updates as information is made available.”
ACT waits for relief
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.
A global air quality index ranked Canberra’s air worse than New Delhi in India or Lahore in Pakistan.