An officer who "leg swept" an alleged Ashburton drunk driver during an arrest used excessive force, the Independent Police Conduct Authority has found.
IPCA chairman Judge Colin Doherty said kicking the handcuffed man's feet out from under him was neither justified nor in self defence.
Police were called to a man reportedly "behaving strangely" and believed to have crashed his car into a bollard aboiut 9.20pm on July 21 last year.
A Southern Communications dispatcher told the officers the driver, 59, was "sitting in a damaged vehicle with hazard lights on".
"He's slurring and got no pants on," the dispatcher said.
The man also had blood on his face.
He failed an alcohol breath test, and the officer told him he would need to go with her to Ashburton police station.
He replied "get f*****". He was eventually forcefully removed from his car and taken to the station.
The IPCA said removing the man from the car was lawful, but an officer's behaviour at the station was not.
Another officer made the man face a wall as he believed he was threatening and intending to "smack" him or spit blood at him.
The officer told the authority he feared for his safety so he leg-swept the handcuffed man off his feet, causing him to land face first on the ground.
On the way down, the man hit his face on a wall heater, making his face bleed heavily.
"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," Doherty said.
The authority said a delay in seeking medical attention for the man was also unacceptable.
Canterbury District Commander John Price said the officer faced an employment investigation under the NZ Police Code of Conduct and was returned to frontline duties, but has since resigned from police for unrelated reasons.
He said the police accepted the force was not proportionate.
"Medical assistance was delayed in this instance due to the focus on obtaining an evidential breath test.
"I also note that the man was provided with the appropriate medical assistance once in hospital."
He said despite the difficult situations they faced, police were expected to behaviour professionally.