A senior police officer tasered eight Armed Offenders Squad recruits in spite of policies forbidding the use of the stun weapons on staff in training exercises.
Police considered a criminal prosecution of the officer, but in the end disciplined the trainer.
Tasers are restricted weapons in New Zealand but are carried as a "non-lethal" tool by frontline police to shock offenders with 50,000 volts of electricity.
Police are tight-lipped about the "inappropriate use" of a taser at the Police College in November last year.
A source told the Weekend Herald about the "appalling use of force" where eight police staff were tasered during a training course to join the AOS.
A different source said the trainer was well-regarded and had a "brain fade" about the change in policy.
But 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.
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.
Without commenting on the specific incident, Police Association president Chris Cahill said police are no longer allowed to taser recruits or staff in training.
The policy changed after the rollout of the taser in 2010 following an earlier trial.
Before the policy changed, a number of police staff - including former Police Association president Greg O'Connor - were filmed being tasered to prove their safety.
Dr Bill Hodge, law professor at the University of Auckland, said the criminal charge being considered by the police would likely be an assault offence.
Consent, implicit or explicit, would be a factor in any decision, Hodge said.
"When you play rugby, there is implied consent to being tackled - which is technically assault. But you're playing sport, so you're consenting to the assault," said Hodge.
"But there's shades of grey too, for example if you get elbowed in the face without the ball in a flagrant way. The implied consent can disappear."
Similarly, even if the AOS gave implied consent to be tasered, this might not be enough.
"There's a power imbalance if the trainer is asking you to be tasered. They're in charge and you want to pass the course," said Hodge.
"And in the macho world of the AOS, I doubt whether anyone would want to say no and be considered a wuss."
Now widely accepted as part of the police armoury now, the introduction of tasers was controversial at the time.
Opponents raised concerns police would abuse the power tasers give them, while supporters argued they are safer - for police and the public - alternative to firearms.
Last year 7500 frontline officers were trained in their use, up from 5500 in 2015.
Police officers can lawfully use tasers when they fear imminent physical harm to themselves or others.
The guidelines say the "necessary, proportionate and reasonable" use is a matter of careful judgment by the officers.
There have still be a number of controversial incidents including a goat being tasered 13 times, as well as the death of a 29-year-old man in Auckland earlier this year.
Alo Ngata was tasered by two police officers after he allegedly assaulted a 76-year-old man.
Ngata, described by one witness as "out of control", died three days later.
In a separate case, an Auckland police has been accused of illegally tasering a woman while on-duty in Auckland.
Sean Mathew Doak, 24, is charged with assault using a taser as a weapon and presenting a restricted weapon "without lawful and sufficient purpose".
But such incidents of death and misuse are rare, according to police statistics.
There were 12 taser complaints to the Independent Police Conduct Authority in 2017, with 2 upheld and 4 others pending.
Tasers were the fourth most common "tactical option" at arrests were force was deemed necessary at (26 per cent*) behind handcuffs (34 per cent), pepper spray (32 per cent) and "empty hand" (39 per cent).
This figure includes simply pulling out the taser, or "laser painting" the target.
Most of the time (84 per cent) this was enough to get ensure compliance.
This means devices were pointed at targets five times more than they were fired, although in 2015, the ratio was seven to one.