A prisoner is in hospital after being "seriously assaulted'' at the Otago Corrections Facility.
A St John spokesman said a 32-year-old with serious injuries was transported from the facility to Dunedin Hospital by helicopter at 10.20am on Saturday.
Facility prison director Dave Miller said staff acted immediately to stop the assault by another prisoner.
"Police have been notified of the assault and are investigating.
"Corrections will also carry out a full investigation into the incident, and will ensure all evidence, including CCTV footage, is provided to police.''
The assaulted prisoner was receiving medical treatment in hospital after being "seriously assaulted''.
"No assault or violent behaviour is tolerated, and all prisoners who resort to this behaviour will be held to account.
"Over 75% of the prison population have convictions for violence in their offending histories, and gang members are disproportionately identified as responsible for assaults in prison.''