The Controlled Purchase Operations are carried out to check processes and ensure compliance with the law by licensed premises.
Disappointingly two premises sold alcohol to these youths, police said in a statement.
"There will be further Controlled Purchase Operations held periodically over the coming months and all licensed premises are reminded to be vigilant," said Sergeant Kyle Bruning, alcohol harm prevention officer.
"A good result for us is when we have no sales made to our volunteers."
Letters to the 18 premises which refused sales will be sent to acknowledge their efforts.
Police said any queries could be directed to Sergeant Kyle Bruning, Nelson Bays Alcohol Harm Prevention Officer on (03) 543 9508.