A Wooroloo Prison Farm inmate injured in an incident two days ago has died overnight in hospital.
Police said this morning the case is being treated as a death in custody.
Major Crime Squad officers are investigating.
Police are trying to determine if the man, a convicted burglar in his 30s, was assaulted or if a medical condition caused him to fall and hit his head.
It is understood another prisoner at the minimum-security jail raised the alarm after finding the unconscious man outside one of the buildings just before 6pm.