A Perth Liquorland employee has told of his shock of seeing usually "unwanted" Pacific Radler beers fly off the shelves on Tuesday afternoon - as Facebook users cashed in on an advertising offer for free drinks.

According to the western suburbs Liquorland worker, scores of Perth drinkers rushed to outlets to grab six-packs after seeing the offer on the Perth Beer Economy Facebook page.

The offer of free beer seemed to have unintended consequences.

The offer apparently originated in an advertisement sent to Perth users of the Weatherzone app and encouraged users to screenshot a barcode and take to their local Liquorland store to claim freebies of the beer, which is two per cent alcohol.

But the promotion took an unexpected turn when a user of the PBE Facebook page elected to share the screenshot with the other 69,000 members of the grog-loving group.