A police sergeant's use of a Taser on a young man was excessive and unjustified, an investigation has found.

The 17-year-old was having an argument with his girlfriend, 18, on the side of a South Auckland road when police pulled over - initially to talk to the couple, before attempting to arrest him just before 1am on September 15, 2016.

In an investigation carried out by the Independent Police Conduct Authority, it found that a police officer's decision to stop and speak to the couple was justified.

His actions to arrest the 17-year-old was lawful and an initial use of pepper spray was justified.

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However, a use of a Taser following that was found to be "excessive and unjustified".

A report shows differing accounts of the incident.

However, all the accounts given show the 17-year-old appears to have become increasingly upset after his girlfriend was pepper sprayed by the officer.

The officer had stopped to talk to the couple when the male teenager called him a "pig sh*t".

On trying to stop her partner from saying anything more, she puts her hands over his mouth - before she is pepper sprayed.

"Mr X [the 17-year-old] said this caused him to 'go nuts', clench his fists and yell at Officer A to apologise. Officer A responded by spraying him in the face."

Two other police officers eventually arrive at the scene.

As those officers work to calm the teenager down, he remains significantly angry at the first officer and spits towards him.

"As Officer A was about to arc his Taser to show Mr X the electrical current as a way to de-escalate the situation, Mr X leant forward and spat again," the report reads.

"Officer A said the spit flew across the left side of his arm.''

As a result, the first officer pointed his Taser at the teen and fired one shot, while calling out: "Taser, Taser, Taser". The teen dropped to the ground immediately.

During the investigation, one of the officers said she was surprised to hear the Taser going off.

"I would not have used a Taser...because it's spitting. It's not going to cause me to end up in hospital. The force is less than a punch or a kick.

"Even though I would have classed it as assaultive behaviour, I would have tried to bring the male down to the ground instead of discharging a Taser."

The report details that the overriding principle when using a Taser is that it can only be used when the situation is beyond assaultive range - actively hostile behaviour accompanied by physical action or intent expressed verbally or through body language.

The first officer said that he took out his Taser when it appeared the pepper spray he had used on the teen did not have an effect on his behaviour and he considered him to be "extremely dangerous".

The 17-year-old was arrested for resisting arrest, common assault and behaving in a threatening manner that was likely in the circumstances to cause violence against persons.

The charges were withdrawn by leave on June 9, this year, as part of the Police adult diversion scheme.

A police statement said an internal employment investigation was still ongoing.

Counties Manukau Police acting district commander, Inspector Julia Lynch, said: "As police have the same privacy obligations when it comes to employment matters as any other employer, I am not able to outline the results of that investigation thus far.

"However, I can say that as with every incident of this nature, lessons have been learnt and we continue to provide training and constructive feedback to help inform decision making and judgement with our staff.''