The boy was taken to hospital with a suspected fractured eye socket.
Police then started a criminal investigation.
The officer was charged with injuring the boy but after two trials in 2020 and 2021 jurors failed to reach a verdict. The Crown decided against proceeding with a third trial.
IPCA chairman Judge Colin Doherty said an independent investigation was completed in 2019, but the report was delayed because of ongoing court proceedings.
"We do not accept that the officer perceived himself to have been at risk of attack from the boy or that he punched the boy to defend himself," Doherty said.
"There was no legal justification to punch the boy and we agree with the police decision to charge the officer with injuring him."
Waitematā District Commander Naila Hassan said police accept the findings of the report, regardless of the charges being dismissed.
She also said an employment investigation resulted in a finding of serious misconduct and the officer is no longer employed by police.
"We place high expectations on our staff and how they perform their duties, and this includes when they are off duty."