Police say the supermarket was caught in a similar operation two months ago.
A supermarket and two other premises in Porirua could have their liquor licenses suspended after being caught selling alcohol to minors.
However, 18 other premises refused to serve alcohol or let young people in if they had no identification.
Senior Sergeant Steve Sargent says two volunteers, aged 15 and 17, visited 14 shops and seven bars and restaurants during an evening.
He says the supermarket was caught in a similar operation two months ago.
The cases have been referred to the Liquor Licensing Authority.