Police used "unjustified" force against a drunk driver who was arrested in Ashburton last year, an Independent Police Conduct Authority (IPCA) investigation has found.
An officer "leg-swept' the man from behind while he was handcuffed, causing him to bleed heavily and sustain facial lacerations and bruising.
Police have acknowledged the IPCA findings and say the officer has been held to account.
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The 59-year-old, referred to as Mr X in the report, was reported to police for behaving strangely and reportedly crashing his car into a bollard on a Friday evening in July 2017.
Police arrived at the scene to find Mr X in his car with blood on his face. After failing an alcohol breath test he was required to be taken to the police station.
Mr X refused to comply and so was arrested by police, who used force to get him out of his car and to the station. This was found to be lawful.
At the station a police officer moved Mr X to face a wall, believing that he intended to smack him or spit blood at him.
The officer said out of fear for his own safety he "leg-swept" Mr X off his feet, causing him to land face first on the ground.
While falling he hit his face on a wall heater, causing him to bleed heavily. He was handcuffed behind his back at the time.
Authority chair Judge Colin Doherty said the officer "did not act in accordance with the law and police policy when dealing with Mr X in that the force he used against Mr X in the charge room was not in self-defence and was unjustified".
In the aftermath of the incident, police focused on conducting an evidential breath test rather than the man's injuries.
The IPCA also found there was a delay in seeking medical attention for Mr X, which was unacceptable.
When he was taken to hospital, Mr X was examined by staff who advised they would keep him overnight. He had sustained lacerations on his cheek and chin which required stitches, and bruising to his left cheek.
Canterbury district commander John Price said police will learn from the findings of the report, and "acknowledge that the use of force in this incident was not proportionate and that medical assistance was delayed".
"Our staff are constantly put in difficult situations, and although these dynamic situations can develop in a matter of seconds, we expect police staff to act professionally," he said.
"It is expected at all times our behaviour and actions model police values. Unfortunately in this matter, the officer did not and has been held to account."
The officer was subject to an employment process and investigation, after which he was returned to front line duties.
He has since resigned from the police force for unrelated reasons.
"The honour to serve and protect our community will remain at the core of our guardianship and we will continue to be the best police we can be," superintendent Price said.