"When the participants failed to hit the required number of targets with their firearms, the course instructor used the Taser by way of a stun contact to their backsides," it said.
"Police conducted a criminal investigation and were unable to obtain sufficient evidence for a criminal prosecution."
"Police also conducted an employment investigation, which the authority reviewed."
The officer was subsequently disciplined as a result of the employment investigation - a decision the authority said it backed.
Some countries allow police officers to be tasered in training, with consent, although this policy was reversed soon after tasers were rolled out in New Zealand in 2010.
Tasers are a safer alternative to firearms, say police, although there have been a number of controversial incidents - including a goat tasered 13 times - despite clear guidelines on authorised use.
The Herald earlier revealed details of the incident after a source called it an "appalling use of force".
A different source said the trainer was well-regarded and had a "brain fade" about the change in policy.
An Official Information Act request for correspondence about the incident, including how decisions were made during the investigation, was declined on privacy grounds.
Instead, Superintendent Anna Jackson released a one-page summary which confirmed police became aware of an issue involving a "trainer and inappropriate use of a Taser".
Jackson, who is charge of investigations into police officers for disciplinary matters, said some of the affected staff alerted a senior officer.
This triggered a Code of Conduct inquiry and once more information was gathered, Jackson said "consideration was given as to whether a criminal charge should be filed".
Jackson said a decision was made not to proceed with a criminal prosecution because of "the overall circumstances of the matter." Advice was sought to assist in making the decision, with reference to the Solicitor-General's guidelines.
Instead, the matter became an employment investigation.
Jackson said the trainer was disciplined but the police cannot provide details because of privacy concerns.
"Police carefully considered the overall circumstances and were satisfied that the outcome was sufficient to take into account the seriousness of the conduct and the assurance the conduct would not be repeated," said Jackson.
"Police are of the view that in a training environment or in any situation where force is being used, that any conduct in breach of our policies and code of conduct, is simply unacceptable."
Jackson declined to comment further or clarify which policies had been breached.
