About 1300 people were evacuated from a Defence office block in Russell after there was a fire in the R1 building.
The Emergency Services Agency said the workers were evacuated as a precaution because of an electrical fire in cabling in roof space.
The fire, on level six, caused minimal damage and no one was injured. Workers have now been allowed to return to the building.
Authorities sent three fire pumpers, the specialist Hazardous Materials Unit, the Breathing Apparatus Van, The Bronto Skylift Hydraulic Platform and a command officer.