Updated 6:00pm: A Cromwell police officer has been charged with assault after an investigation into police brutality at the local police station.

Relieving Area Commander Olaf Jensen says they have completed their investigation into the allegation that a constable used excessive force earlier this year.

He says the constable remains on duty but will be restricted from undertaking frontline work while it's being dealt with by the courts.

The Independent Police Conduct Authority is also investigating.