Victoria Police have offered a $350,000 reward for information that leads them to a masked gunman who shot a convenience store worker at close range during a bungled holdup in Melbourne's south-east.

New CCTV shows the disturbing moment a shooter opened fire on an attendant working at a Springvale South 7-11 as he desperately ran for cover in a storeroom just before 4.30 am on August 8 last year.

Police admitted they have "exhausted their avenues of inquiry", and are calling on anyone with information to come forward.

The offender fired a shot over the counter hitting the 7-11 employee in the arm. (9News)

He tried to access the till himself but was unsuccessful. (9News)

While the doors to service stations are commonly kept locked during the night, the offender gained access by trailing a customer.

The masked man threatened the employee, who was behind the counter, making demands for money, before shooting the man in his arm with a silver-coloured handgun.

He then attempted to remove cash from the till, but was unsuccessful and fled the scene.

The victim was taken to hospital and underwent surgery, with police saying he has required ongoing medical treatment and continues to suffer psychologically.

"The victim has since suffered serious psychological trauma as a result of this incident, and hence he's not here today to be part of this announcement," Detective Inspector Dean Thomas said today.

Det. Insp. Thomas described the offender as being of African appearance, wearing a grey-coloured windcheater, dark-coloured Adidas tracksuit pants and white Adidas shoes. He was wearing a black beanie and his face was covered with a black and white bandana.

Police have been unable to break the case and are calling for anyone who knows the offender. (9News)

Police believe the offender was involved in another attempted armed robbery on the same night at a service station in Heatherton Road in Springvale near the corner of Hoskins Street.

"Going by the distance and time I'm confident that there was a vehicle involved," Det. Insp. Thomas said.

Police believe there was at least one other offender driving a vehicle but do not have a description of either.

Concerns have been expressed about the propensity of the offender to use the firearm unprovoked, with police saying it's "obviously a concern for the Victorian community".