Multiple firefighting units and a water bombing helicopter were called to a grass fire burning at Cooleman Ridge near Kambah on Wednesday.
The Emergency Services Agency said the fire was about 500 metres by 150 metres, burning at the southern end of the nature reserve.
It was under control by about 5pm, and no properties were at threat.
By 6pm, ACT Fire & Rescue crews had departed the scene, while 11 units and an officer from the ACT Rural Fire Service were expected to remain for another few hours to mop up the fire.
The fire was about 600 metres from Allchin Circuit, Kambah.
The bushfire alert and warning was set at advice.
ACT Fire & Rescue and the ACT Rural Fire Service had 15 units on the scene and a water bombing helicopter at its peak. Two community fire units in the area were also there.
The area will be patrolled overnight to ensure there are no flare-ups.