An equipment fault, leading to a build-up of unsafe carbon monoxide levels in the Department of Justice and Community Safety building, was the cause of an emergency evacuation in Wangaratta's CBD on Thursday afternoon.

Fire Rescue Victoria (FRV) received a call from department staff at 1.34pm stating a carbon monoxide sensor was operating in the basement of their building on the corner of Murphy and Faithfull streets.

Within six minutes, firefighters were on the scene and conducted their own readings, which showed levels of carbon monoxide were unsafe and the building needed to be evacuated, which Victoria Police assisted with.

FRV's hazmat specialists and a scientific advisor determined a specialist extraction system within the building had stopped working and consequently caused the leak.

Authorities said a cleaner had been using a petrol cleaning product in the basement, which had caused a build-up of carbon monoxide.

DJCS staff worked with FRV crews to restart the extraction system.

Crews then successfully executed a plan to safely ventilate the area, with minimal exposure to firefighters, residents and workers.

The scene was declared under control shortly before 3pm.

FRV incident controller commander Sim Michie said this was a great example of emergency services working together to keep the public safe.

"It was a great effort by Ambulance Victoria, CFA crews, FRV and Victoria Police - as well as the DJCS staff - to ensure nobody was sick as a result of this incident and everyone got home safely," he said.