As dozens of people were forced away from the intense flames, it was Damian Holloway's job to run towards them.

The six-year veteran of ACT Fire and Rescue was returning from a faulty smoke alarm at Canberra Airport when he and his crew saw flames on the city skyline.

He quickly became one of the first people called on to fight the flames.

''When we pulled in there was a lot of smoke coming out of the top of the building and we could see that one of the shopfronts had its windows and door blown out,'' he said on Tuesday.

Along with his crew mate, Mr Holloway grabbed tools to gain entry to the burning shopfront and began searching for the fire's source.

''Your heart starts beating and you get a fair adrenalin rush,'' he said. ''You have to concentrate on keeping calm … and there's a lot of communication with the person you are working alongside. You just go from there and try to attack.''

The first emergency call came at 9.46am, reporting a fire inside Coo Japanese restaurant.

Mr Holloway and his colleagues quickly pushed through acrid fumes, exploring as the fire engulfed three storeys.

''You have zero visibility in that amount of smoke,'' the 32-year-old said. ''As we were having a look, we could see there was a large area of heat in the ceiling, indicating to us that the fire was actually above us as opposed to in front of us and below us.''

Using a process of elimination and assisted by arriving back-up crews, Mr Holloway relied on tanks of oxygen lasting about 25 minutes.

With a racing heart and struggling to be heard through a hand-held radio, he had to refill three times.

''As the job was progressing, the smoke and the intensity got a lot worse, to the point where it was starting to back-draft.''

Hundreds of people watched on from nearby streets and buildings, with social media flooded with photos of rising smoke.

''A building of that size is bigger than a house fire which you might usually go to,'' Mr Holloway said.

''House fires can vent a lot quicker, which means visibility can improve a lot quicker. We were dealing with a much smaller area of intensity and it was tough going.''

Crews eventually withdrew to a defensive position, fighting the fire using an aerial unit and breaking through the tiled roof.

Nearby businesses were evacuated, with many set to remain dark on Wednesday.

''The smoke changed colour and became quite turbulent, which was indicating that the chance of back-draft or flashover were increasing, which is where you have a large amount of heat built up.

''It took several hours before it was at the point where the fire was under control.''

It was not until about 2am on Tuesday that crews completely extinguished the blaze and air monitoring could begin.

''I would be lying to say it's not exciting,'' Mr Holloway said.

''It is terrible to see a building burn down or someone lose their home but your adrenalin does lift when you see something like that and there is excitement that comes with that.''

ACT Policing took control of the site on Tuesday afternoon, starting the process of forensic investigations into the cause.

Incident Commander Michael Hobbs said Monday's fire ranked alongside the gutting of the Diamant Hotel in terms of complexity and would be extensively studied in coming weeks.

''You can't apply one rule but generally fire is predictable and like water in many respects,'' he said.

''It can always find a sneaky path and when it's not vented it moves sideways in the compartment and that's why it was coming out the front and out the back door here.''

Mr Hobbs said crews had done an outstanding job in limiting the blaze to a small part of the 1920s structure.

Fire investigator Anthony Walker said structural issues were slowing the investigation.

''If this turned out it was actually an explosion, we will be considering what the fuel source was and what triggered it," he said. ''We work through what we have at the end and go backwards to what was the cause."

Investigators will talk to each firefighter who tackled the blaze to assemble a complete picture of what took place.

It is expected the analysis at the Sydney Building will take a few days.

As local traders and insurers take stock of the damage, Damian Holloway is blunt about his role in the events that have captured Canberra's attention this week.

''You join the fire brigade to put out fires,'' he said, smiling. ''It's just what we do.''