A gunman fired about five shots inside Canberra's main airport, sending passengers fleeing but injuring no-one before he was detained by Australian police.

The Australian airport was evacuated and locked down, leading to the suspension of flights.

Images posted on social media showed a police officer restraining a man on the ground inside the terminal as the emergency alarm sounded in the capital's main airport.

"A male has entered Canberra Airport in the departures area. He has sat in one of the areas adjacent to the glass windows," detective acting superintendent Dave Craft told reporters outside the airport building.

"After approximately five minutes, this male has removed a firearm from his possession and let off approximately five rounds," he added.

Supt Craft said the crime scene indicated that the man had fired shots at the glass inside the terminal.

Several apparent bullet impacts were visible on the glass front of the airport

"There was no shots directed at people, or persons, passengers or staff," he said

A witness was quoted as telling a reporter for The Guardian newspaper that she saw a man "shooting into the air" not far from the check-in counter, describing him as being middle-aged and "clean cut".

Police said the man was taken into custody and a firearm was recovered. He was being detained at a police station in the city.

After examining the airport's closed-circuit television images, police said they believed the man had acted alone.

The situation was now "contained", police said, describing the airport as an "active crime scene".

Police said that the motive behind the shooting was not immediately clear.

Prime Minister Anthony Albanese said he had been briefed on the shooting incident.

"I am advised a man has been detained and there is no ongoing threat present," the prime minister said in a statement.

People wait after being evacuated from the terminal

ABC journalist Lily Thomson reported that she heard the shots before people started to scream.

She saw a fearful woman looking after a baby.

"So we all ran and I stayed with that grandma and her baby and hid behind an information desk," Ms Thomson reported on ABC.

"We stayed there for a couple of minutes until security told us to evacuate out to the car park," she added.

"Everyone was hiding behind chairs and people were running."

Canberra airport said it was working with airlines to restart flights by this afternoon, but some flights had been cancelled.