Neither police nor the Department of Corrections would confirm if the prisoner was assaulted, but both authorities were investigating an "incident".
St John took the man, aged in his 20s, to Dunedin Hospital with serious injuries after the incident yesterday morning.
Emergency services were alerted about 11.20am.
"Police were made aware this morning of an incident at Otago Regional Corrections Facility, and are making further inquiries," a police spokeswoman said.
No-one had been arrested in relation to the incident at present.
When asked if the man’s injuries were the result of an assault, police said they were "not in a position to elaborate further at this stage".
OCF director Lyndal Miles also refused to confirm how the prisoner received his injuries, but said in an emailed statement Corrections had a policy of "zero tolerance to violence".
"Corrections can confirm that a prisoner has been taken to hospital this afternoon following an incident at OCF," he said.
"The prison is currently investigating what happened and no further details are available at this stage.
"No assault or violent behaviour is acceptable. However, it is an unfortunate reality that from time to time this will occur.
"Any prisoner who resorts to violence will be held to account.
"We manage some of New Zealand’s most difficult and challenging citizens and violence is always a risk as many offenders resort to violent behaviour as a means of resolving issues and expressing themselves."
