A liquor-store attendant ignored the knife a drug-addicted teenage boy was holding in his face and calmly talked him into leaving empty-handed, say police.

The robber entered the Levesque Liquor Store in Birkdale on Auckland's North Shore, last Friday.

Police said that he held the cool clerk at the point of a serrated knife, threatening to hurt him unless he gave him alcohol and cash.

The attendant calmly engaged the boy in chat, during which his potential attacker robber told him he was 15, and needed the money to buy the drugs he was addicted to.

The attacker left, empty-handed, soon after, North Shore Detective Constable Laura Ellis said.