Police are copping another round of criticism over their taser use.

They've again been faulted by the Independent Police Conduct Authority, this time for using one on a man who was mentally unwell.

This is the fourth incident since October last year that the IPCA has found officers guilty of using their tasers unlawfully, three of which have been from this year.

This week the IPCA found that the South Auckland sergeant's use of a Taser on a mentally unwell man at the Counties Manukau Police station was "contrary to policy, excessive and unjustified".

The victim was in a small cubicle at the Police Station in April last year when it was deployed.

The man had been picked up by police after his family said he had been cutting himself earlier in the day with a small pocket knife and that it was possible he still had this knife concealed on him.

Officers arrested the man for possessing an offensive weapon and took him to the police station. He was told he would be strip searched and was taken to the cubicle to wait for officers.

He refused to remove his clothes and police advised him that if he did not, they would cut the items off him.

As they tried to remove his clothes the man resisted.

A sergeant entered the cubicle and used a Taser twice on the man, his actions caught on the room's CCTV camera.

Footage showed the man was being held down by two officers, and had his back turned to the officer with the Taser.

The Tasering was reported to the IPCA, which investigated and released its findings today.

Investigations found police policy clearly says a Taser can only be used against a person who's being abusive.

The report also said that the officer could have handled the man in a less violent way. 

Police say they accept the findings, and that they could've handled things differently.

They say a number of lessons have been learned.