Department of Corrections southern regional commissioner Ben Clark confirmed the three men were set to appear in court yesterday on joint charges, before two of the men allegedly assaulted the other man.
Duty lawyer Max Winders said the trio were serving prisoners at the Otago Corrections Facility and were due to appear yesterday, along with the other man, in the Dunedin District Court on a charge of injuring with intent to do grievous bodily harm, following an alleged incident at the prison on April 1.
Ambulance officers were seen just before 9.30am carrying a stretcher into the cell entrance to the courthouse in Castle St.
About 10 minutes later, a young man who appeared to be unconscious was carried into the ambulance on a stretcher with his head in a brace.
St John spokesman Gerard Campbell said a man with serious injuries had been transported from the court.
A Corrections Officer escorted the injured man to Dunedin Hospital.
Ministry of Justice general manager of health, safety and security Melissa Gill confirmed there was CCTV operating in the cells area of the courthouse and the footage would be reviewed as part of the investigation.
She confirmed the men involved were in the custody of Corrections officers when the alleged attack took place.
Earlier, people could be heard banging on the internal walls of the police custody van as it drove towards the building.
Police prosecutor Sergeant Mike Wingfield said the Criminal Investigation Branch would interview the two men involved in the alleged assault with a view to further charges being laid.
In court, Mr Winders suggested all three should be remanded until Tuesday without appearing. Police supported that stance.
Judge John Macdonald said if there was a security risk posed by the defendants, it would be ''classic case'' for them to be called by audiovisual link, rather than brought to court again.
-Additional reporting by Rob Kidd
