Police say a bullet struck the sign at the top of the government van about 9.40pm last Monday, when it was parked on the median strip of Canberra Avenue, near the intersection with Eyre Street and Lefroy Street.

Detective Acting Inspector Adam Liversidge said a gun had been seized on Thursday night and it was being forensically examined to see if it had been used in the incident.

CCTV footage shows a silver station wagon driving past at the time of the incident.

Detective Acting Inspector Adam Liversidge said the public might not realise speed vans always had operators inside.

"ACT Policing worked closely with Access Canberra when the incident was first reported to ensure there was no ongoing risk to safety," he said.

"No one should be put at risk doing their job from reckless behaviour like this. All speed vans have high-quality CCTV around their vehicles, which police are reviewing as part of their investigations.

Police say the weapon seized is not the same as AFP issued firearms, after a glock pistol was stolen from a police vehicle several weeks ago.

Police are appealing for anyone who witnessed or might have information about the incident, or who may have dash-cam footage from the area to come forward.

Anyone with information that could assist police is urged to contact Crime Stoppers on 1800 333 000, or via the Crime Stoppers ACT website and quote reference 6646275. Information can be provided anonymously.