Firefighters brought the Alderson Place emergency under control by about 1.30pm. Three trucks, a HAZMAT unit and a firefighting crane were on the scene. Holes were cut in the roof for ventilation, as crews battled the blaze from the outside.

Security guards remained on-site overnight with fire investigators to return to the scene Thursday morning to determine the cause of the blaze.

ACT fire commander Michael Cochrane said firefighters first responded about 11am.

"There seems to be a fairly high fuel load inside the structure," Mr Cochrane said.

Earlier, Mr Cochrane had pulled out crews inside the building over concerns of those high fuel loads, but by 1pm fire crews had re-entered.

Bystander Hugh McKenzie and his mum, Louise McKenzie, saw a man trying to fight the fire with a garden hose.

"He walked out of the back there and he just sort of laid down and then he got back up again. Everyone went down to help him," Mr McKenzie said.

"I haven't really seen anything like that before."

The man was seen to sit up, before laughing with the firefighters and paramedics as they treated him.

He stood up and walked with paramedics, and people who appeared to be family or friends, to an ambulance.

Emergency services confirmed the man had a cardiac episode before he collapsed but was being treated at the Canberra Hospital.

ACT Policing managed local traffic and road closures. Alderson Place in Hume was closed until further notice.

Smoke was visible in several nearby suburbs, including over the border in Jerrabomberra and Queanbeyan.

The ACT Health Directorate warned that people with respiratory conditions should avoid exercise and stay indoors if affected by the smoke.