A family milk bar in Melbourne's outer west has been robbed at knifepoint for the second time in a month.

Police believe up to eight youths wearing bandanas and balaclavas stormed the Riverdene Village Shop in Werribee about 7.30pm on Sunday.

The terrifying attacks have rocked the family who run the milk bar and are closing down the business.

It's believed the group jumped over the counter and stole cigarettes and cash while causing significant damage to the Derwent Road store, smashing windows and pulling items from shelves.