At 11.10am (Wed 13 Jun), 10 CFA units were called to the Gateway Plaza in High Street after complaints of a metallic smell and people feeling nauseous.
The Corio Hazard Detection Unit used atmospheric monitoring equipment to try and determine the cause.
CFA issued an Advice message just after 12pm alerting people of a hazardous material in the area.
Ambulance Victoria treated 13 people at the scene but no one was transported to hospital.
The incident was under control by 1.24pm and several people were allowed to return to the shopping centre.
CFA was unable to determine the cause of the smell but is still investigating.
An industrial hygienist, in conjunction with WorkSafe, will monitor the area overnight.
Please see the Melton Weekly story for photos from the scene and an interview with Operations Officer Trevor Roberts.