Brimbank Crime Investigation Unit detectives are appealing for witnesses following an armed robbery at a post office in Deer Park on Saturday.
Police have been told that a man entered the post office at a shopping centre on Neale Road just before 10.30am.
It is believed the man approached the counter and made demands for cash, threatening a female staff member with a firearm.
The offender jumped over the counter and stole cash from three cash registers.
He then fled the scene and was last seen running south towards Neale Road and Gould Street.
He is described as being in his 20s and was wearing a black jacket with hood, grey track pants and thongs.
Police have released an image of a man who they believe can help with their inquiries.
Anyone who witnessed the incident or who has further information is urged to contact Crime Stoppers on 1800 333 000 or visit www.crimestoppers.com.au