A faulty boiler caused carbon monoxide to leak into Woden Plaza today, causing two people to collapse, and hospitalising four more.

ACT Fire & Rescue cordoned off an area in Bradley Street, Woden, for much of this afternoon, after a man and a woman collapsed just after midday.

They were taken to The Canberra Hospital in a stable condition, while a further four people were later taken for observation as a precaution.

Emergency services blocked off a small area between the McDonalds entrance to Woden Plaza and Bonner House, just across the road.

ACT Fire and Rescue dispatched hazardous materials crews to conducting atmospheric monitoring in the area, before lifting the cordon about 4pm.

Further investigations revealed that a faulty boiler may have released carbon monoxide into the air conditioning system of Bonner House. 

Crews have ventilated the building, and declared the area safe shortly before 4pm.Work safety authorities will now investigate the incident.