The airport was quickly evacuated with hundreds of people standing outside the building.
More than a dozen police vehicles are stationed outside the airport while armed offices guard the outside of the building.
There are no reported injuries.
Unconfirmed reports suggest a man with a revolver shot rounds into the airport windows near the Qantas desk.
It is understood he had not passed through the security checkpoint.
Images posted to social media show bullet holes and smashed glass panels where the gunman allegedly aimed the shots.
Witnesses say the AFP is conducting a security sweep while the airport is evacuated.
A Canberra airport spokesperson declined to comment, saying it was a police matter.
No planes are understood to have been diverted, but none have left the airport since the shooting.
Flights that arrived around the time of the shooting have been taxied on the tarmac, according to flight radars.
Police are asking members of the public not to attend the airport in the coming hours, but flights are expected to resume this afternoon.
