Firefighters were called to Ward 9A at the hospital about 10am yesterday after a mattress was ignited.
The medium-security ward contains patients convicted or charged with a criminal offence who suffer, or are suspected of suffering, mental illness.
It is understood the ward contains several patients with current or prior arson allegations.
Firefighters from Willowbank, Roslyn and St Kilda attended the blaze and discovered a ward with ''extreme smoke-logging'', Senior Station Officer Pete Douglas said.
A mattress with a plastic cover had been ignited, although staff in the ward had extinguished it before firefighters arrived.
''We encountered a few [people] who had been in the toxic smoke for a short time,'' he said.
''St John assessed eight staff for smoke inhalation ... four were taken to hospital.
''One person from the facility was looked over by St John.''
A St John spokesman said one person with moderate injuries and three others with minor injuries were taken by ambulance to Dunedin Hospital's emergency department.
Southern District Health Board mental health addictions and intellectual disability directorate general manager Louise Travers said staff and patients were ''safe and well'' following the fire.
''Eight staff were checked by St John ambulance, three of whom were taken to Dunedin Hospital ED as a precaution to be assessed for smoke inhalation,'' she said.
''A fourth member of staff attended the ED later in the day after becoming unwell.
''One patient was checked by St John and medical staff at the scene, and determined to not have any injuries. No-one else was harmed as a result of the incident.
''Both Ward 9A and 9B were back to business as usual within a very short time of the incident.''
Senior Sergeant Ben Butterfield said the fire was suspicious and the Dunedin CIB was investigating.
''No arrests have been made and the investigation is ongoing,'' he said.
