Police admit the situation where a taser was used on a 17-year-old who spat at an officer could have been handled better.
The Independent Police Conduct Authority (IPCA) found the use of the taser was excessive and unjustified.
The IPCA said the sergeant used the taser after the 17-year-old spat twice in his direction in South Auckland in September last year.
The incident occurred after the sergeant pulled over to speak to the man and his girlfriend who were having an argument on the side of a road in Manurewa.
IPCA chair Judge Sir David Carruthers said the man was being restrained by two officers at the time he was tasered.
Judge Carruthers said police policy clearly states a taser must only be used on a person who is assaultive.
Police said although the man was resisting arrest and verbally abusing the sergeant, the incident could have been handled better.
"The officers were attempting to handcuff the man who was once again resisting arrest and then spat twice in the direction of the Sergeant," a statement said.
"It is at this point that he has used his Taser on the man.
"The man did not make a complaint regarding the incident and it was Police staff who referred the incident to the Authority.
"Although the man was resisting arrest, was verbally abusing the Sergeant and spat twice in his direction, police acknowledge that the incident could have been handled better."