Australia Post has suspended all deliveries in and out of Canberra due to the hazardous smoke conditions. Six post offices in Canberra were closed.

The National Australia Bank closed all of its Canberra offices on Thursday.

The Canberra Times understands that many businesses in Fyshwick had sold out of P2 masks as well as Bunnings' stores across Canberra.

According to the US air quality index, Canberra had the worst air quality of the major cities in the world with readings higher than Beijing in China and New Dehli in India.

Questacon director Professor Graham Durant said the centre had to close on Wednesday afternoon due to poor air quality inside the building.

"We are a very open institution and every time the doors opened with people coming in, it got increasingly smoky," he said.

Professor Durant said the smoke was setting off the institution's automatic alarms and they were forced to switch them off.

"That's one of the reasons for making the decision as we would have had no protection in the building," he said.

Questacon will make a decision at 8am every day in the coming days about whether or not to open.

Cockington Green Gardens was also forced to close its doors for a second day.

"This is certainly not our preferred decision, but we need to recognize what is happening around us and consider the welfare of our staff as well as the well-being of our visitors," Cockington Green director of operations Mark Sarah said.

"These are exceptional circumstances with highly hazardous conditions that need to be considered and as such our position is to monitor the situation on a daily basis."

The National Museum of Australia said it was closely monitoring the situation.

"For the health and safety of our staff and visitors we will make a decision each morning at 8am on whether to close the building. We will be communicating any closures through the museum's social media channels," a spokeswoman said.

The Canberra Hospital has confirmed some MRI Machines have been affected by the smoke and are unable to run.

The following pools are closed on Thursday:

Canberra Business Chamber chief executive Michael Schaper said for seasonal businesses, such as pools, the effect of the haze could be financially devastating.

"The whole year's income can be riding on a couple of weeks," he said.

The smoke has blown in from fires on either side of Canberra with the South Coast fires in the east and the Dunns Road fire, near Mount Kosciuszko in the west.

The air quality rating peaked at 5185 at 8pm on Wednesday night in Canberra's south at the Monash air quality station which is 26 times above hazardous levels.

As of 2pm on Thursday, the Monash air quality station had an index rating of 2474. In the north, the Florey air quality station had a rating of 2060 and the Civic air quality station was 2117.

Bureau of Meteorology forecaster Abrar Shabren said the smoke haze would linger in Canberra for the coming days with visibility expected to be poor.

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At the Canberra Airport on Thursday morning, there was 600 metres of visibility, on a clear day visibility from the Bureau of Meteorology's radar is 10 kilometres.

Despite the low visibility no Qantas aircrafts had experienced a delay in departures of arrivals at Canberra Airport.

Virgin aircrafts had not experienced delays either due to smoke haze, a spokesman said.

The bureau has issued a dangerous road alert for Canberra due to the poor visibility. ACT Police urged drivers to be cautious with the smoke haze

"Drive to conditions, turn your headlights on and just slow down," superintendent Jason Kennedy said.

"Avoid any travel if you don't need to."

Dangerous conditions have also forced the closures of the following parks and reserves until Sunday:

On Wednesday Canberra recorded the worst air quality rating since records began 15 years ago.

ACT Health have issued multiple warnings.

"During these periods of heavy smoke, all Canberrans are advised to avoid physical activity outside," acting chief health officer Dr Paul Dugdale said.

"We are strongly advising people who are sensitive to smoke, especially those with pre-existing heart and lung conditions, to take extra care during these conditions."

Australian Medical Association ACT president and Canberra GP Antonio Di Dio said the relentlessness of the smoke was making some people anxious.

He said people should take precautions, but it was important not to become obsessed about the smoke from a health perspective.

"People need to be aware about their own health and those very young and very old," he said.

"Also be very careful if they've got asthma and lung disease or they're a smoker. Be careful about going outside if it's not necessary to do so."

Doctors have been seeing more and more patients affected by the smoke haze.

"There are more coughs, more asthma, more shortness of breath and exacerbations of known respiratory conditions," Dr Di Dio said.

He said people with mild asthma were increasingly affected.

Asthma Australia senior manager programs Nigel Cooper said the smoke haze could cause an inflammatory response in people with low or controlled asthma that could last weeks.

"It take a while for the swelling [from inflammation] to come down again," he said.

"Be really prepared these are unprecedented conditions for people who think their asthma is well controlled."

Mr Cooper said asthmatics should not go outside and should avoid any unnecessary travel.