Dangerous levels of carbon monoxide are thought to have leaked from a faulty boiler in Bell's Dry Cleaners in Bonner House, adjacent to Woden Plaza, on Sunday morning.

The toxic gas leaked into the building's air-conditioning system, and flowed to a neighbouring alterations shop, investigations suggest.

A man and a woman working in the shop collapsed about midday.

It is not known how long the pair may have been inhaling the toxic gas before they passed out.

A security guard who went to the aid of the workers also passed out.

Emergency services were called to the scene and a paramedic went into the shop to help the trio.

The ambulance officer was also affected by the carbon monoxide, and required treatment from his colleagues for nausea. Two other people were taken to hospital for observation.

ACT Fire and Rescue quickly cordoned off the area around Bradley Street near McDonald's, closing it off for much of the afternoon. Firefighters shut down all equipment in Bell's Dry Cleaners and ventilated the building, while ambulance officers took six people to Canberra Hospital, two of whom were expected to stay there on Sunday night.

Work safety officers have now taken over the investigation and will attempt to confirm the faulty boiler was responsible. They have shut down work at the dry cleaners and placed a prohibition notice preventing the boiler from being used.

Work has also been prohibited at the alterations shop.

Work Safety Commissioner Mark McCabe told Fairfax Media the incident was unlike any he had seen before.

''It certainly sounds like we're lucky to have avoided much more serious injuries,'' Mr McCabe said.

''We're still trying to get to the bottom of this. We've got some educated guesses about what has gone on. Our staff will be back out there [Monday] morning to continue the investigation.''

He said it was unclear whether the leak could have been prevented.

ACT Fire and Rescue crews monitored the air on Sunday, lifting the cordon about 4pm.