Young staff at a Subway franchise in Melbourne's western suburbs are traumatised following an armed robbery by a gun-wielding, bicycle-riding thief.

The man entered the family-operated franchise on Ashley Street at Braybrook, about 8.40pm on November 24 and threatened workers with a gun. They handed over cash before the man fled on a bicycle.

A CCTV image of the man robbing the Braybrook Subway store. Credit:Victoria Police

Detective Sergeant Damien O'Mahoney says the incident has left them shaken.

"The victims of the Ashley Street incident were younger people – a male in his late teens and a female in her early 20s – and they are traumatised by the experience," he said on Thursday.