A gun-wielding thug has been brought undone by a jar of instant coffee.

Jason, who did not wish to use his surname, was buying coffee and cigarettes at his local service station in the Melbourne suburb of Frankston on Tuesday when a man armed with a replica pistol asked for his keys.

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When he refused, Jason was struck over the head and fell to the ground.

His fresh jar of Nescafé Blend 43 smashed, allowing him to use the shards of glass to fight off the attacker.

CCTV caught the extraordinary incident and Jason joined Sunrise to detail exactly what happened.

Police wish to speak to a caucasian man, about 183cm tall, slim, and aged in mid-20s. Anyone with information is urged to contact Crime Stoppers on 1800 333 000