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A Fire and Emergency New Zealand (Fenz) spokesman said firefighters were called to the hospital about 10.30am.
"Multiple fires" had been lit in the facility, the spokesman said.
In a statement this afternoon, Louise Travers, the Southern DHB's General Manager, Mental health, Addictions and Intellectual Disability, confirmed the fires had been lit by a patient.
She said the fires were all extinguished by hospital staff.
"Staff followed our protocols for such events. In this instance this included safely extinguishing the fires as they were able to do so, alerting Fenz, having non-essential staff leave the premises and safely managing our patients in the building."
She said the incident would be logged and investigated "as part of our usual processes".
The Fenz spokesman earlier said they were also investigating.