Brimbank Embona Taskforce detectives have released a facial composite image as part of their ongoing investigation into a series of armed robberies in Deer Park and St Albans.

Investigators believe the incidents, which have taken place in the past five weeks at a service station, two pharmacies and a milk bar, may be linked to the same offender.

The first incident, at a chemist on Station Road, Deer Park, involved a man who threatened two people with a firearm on May 21 about 7pm.

On June 18, a female attendant at a service station on Neale Road in Deer Park was threatened by a man brandishing a firearm and demanding cash about 6pm.

A third incident occurred on June 19, about 4.30pm, when a man with a firearm threatened an employee at a chemist on Main Road West, St Albans.

On July 10, an employee at a milk bar on Porter Court in Deer Park was threatened with a firearm about 12.30pm.

Investigators have released CCTV vision, as well as a facial composite image, of a man that may be able to assist with their inquiries.

He is perceived to be Caucasian in appearance, aged in his early to mid-30s, about 183cm tall, with olive skin, brown eyes, a medium build and a short beard.

Police believe the offender is likely to be local to the Brimbank area.

Anyone with any information is urged to contact Crime Stoppers on 1800 333 000 or submit a confidential crime report at crimestoppersvic.com.au