The fire was extinguished in less than 15 minutes after ACT firefighters responded to reports of the blaze at about 11.45am, according to the Emergency Services Agency.
A spokesperson said police extinguished a small fire at the front of building.
The building was evacuated as a safety precaution.
The scene will soon be handed over to ACT Police to investigate.
A large number of police are guarding the building as protesters, who are bearing an Aboriginal flag, stand nearby yelling.
Melbourne chef Adrian Richardson, who witnessed the blaze, has told 3AW he saw the front balcony above the front steps burst into flames.
He said protesters near the site were chanting “let it burn”.
“There’s a lot of police, there’s a lot of police with flags all waiving and screaming, a lot of fire trucks,” he told Shane McInnes.
The cause of the fire is unknown.