The canisters explode soon after reaching the ground, setting alight the fuel that has grown in the Namadgi National Park, south-west of the city. Territory and Municipal Services on Wednesday began the largest controlled burn in the ACT in more than three decades, covering 6000 hectares in an area that was damaged by the 2003 bushfires.

Fire management unit manager Neil Cooper said that a decade on, staff measurements indicated fuel had built up again, vegetation had recovered enough to withstand a low-intensity burn and a burn-off was needed to help prevent another disaster.

''[The 2003 fires] is something that we're very mindful of because there's some regenerating vegetation up there that's sensitive, so we've had to plan this burn around a number of threatened flora and fauna species,'' he said.

The fire is bounded to the west by the Corin Dam, to the north by Corin Dam Road, to the east by Smokers Trail and to the south by Cotter Hut Road.

Mr Cooper said autumn was the perfect time for burn-offs, the days were getting cooler and wetter and smouldering logs would eventually go out.

This week's burn was still more than a year in the planning, and expert knowledge on lighting patterns was used to predict and control how fires would burn.

''Around those sensitive areas we pick the evenings to light and work out rates of spread, so by the time the fire gets to a sensitive area it's probably about midnight and will go out by itself anyway,'' he said.

The aim was not to destroy everything on the landscape, but rather burn a mosaic into the area, like those created by lightning strikes and small fires.

''We're trying to mimic nature a little bit … so when a wildfire comes along, and it will, we will certainly get more fires, there will be areas where there's not that much fuel, so it will drop to intensity where we can get in there and suppress it,'' he said.