One of the premises served alcohol to a 15-year-old who presented ID clearly showing their age, Inspector Davey said.
The result is similar to other recent sting operations in the Auckland City Police District.
One operation in March saw five out of seven bars serve alcohol to 16 and 17-year-olds.
Police targeted premises in Auckland's western suburbs in the March operation, particularly Mt Eden and Pt Chevalier.
In April, four out of eight premises targeted in Ellerslie, Panmure and Newmarket sold alcohol to minors, one as young as 15.
Inspector Davey said the four failing premises in the latest sting would be "dealt with in the strongest possible way".
Applications for suspension or cancellation of licences and duty manager certificates for the failing premises will be submitted to the Alcohol Regulatory and Licensing Authority, police said.
Statistics released in October 2015 showed the Auckland City Police District had the highest incidence of premises selling alcohol to minors in the country.
Nearly a quarter of Auckland premises that were targeted failed the police sting operations over the previous year.
Most of the 285 businesses targeted were off-licence premises such as supermarkets and dairies.
62.6 per cent of on-licence premises in Auckland, including bars, pubs and restaurants, served alcohol to underage patrons.
The average for both on- and off-licences in the rest of the country was 6.1 per cent.
Auckland City District Police plan to continue the operations.
