A man purporting to purchase flavoured milk pulled out a sawn-off shotgun from underneath his shirt at a service station counter before fleeing with cash and cigarettes in a terrifying hold-up in in Melbourne's outer south-east.

The man, clad in a Furphy beanie and a Nina and Pasadena brand t-shirt, entered the Tooradin service station, on the South Gippsland Highway, about 3.15am on Tuesday morning.

The man, in his 30s, then strolled over to the fridge and selected what appears to be a strawberry-flavoured Big M before approaching the shop's the front counter.

He then asked the attendant for a packet of cigarettes, producing the sawn-off shotgun from underneath his shirt as the attendant's back was turned.

The offender then fled the scene after the frightened attendant handed over money and tobacco.

CCTV footage depicts a man who appears to be Caucasian, aged in his 30s with a slim build and wearing a black zip goodie, a black t-shirt with 'NXP' written on the front and jeans.

Detectives from the Casey Crime Investigation Unit are urging anyone with information to call Crime Stoppers Victoria on 1800 333 000.