The operation resulted in four sales from 18 outlets, two from the same store.
Sergeant Pauline Jones, alcohol harm reduction officer of Rotorua police, said the result was "extremely disappointing".
"No one under the age of 18 should be able to purchase alcohol and we expect the three premises who did sell alcohol to our volunteers to have better systems in place to prevent this occurring in the future.
"As part of the process to obtain a licence to sell alcohol, applicants receive training and are interviewed to ensure that they have a clear understanding of the act," she said.
"There is an assurance that there are appropriate systems and procedures in place to prevent the sale of alcohol to underage people."
She said the undercover operations were not designed to catch people out.
"They are necessary to monitor compliance with the act. Police consider a successful operation to be one where our volunteers are not able to buy alcohol at any of the premises.
"I would like to thank the other participating outlets that did comply with the act and refused to provide the volunteers with alcohol."
The Rotorua Daily Post asked whether there would be any censures for the three premises that breached the act.
"Police will work with the premises concerned to ensure they have the support necessary to put good processes in place and avoid supplying minors with alcohol in the future," Ms Jones replied.
The police also declined to name the three businesses.
