Police have been called out to Hillmorton Hospital to a serious assault.
The incident happened about 9.30am at the mental health facility in Christchurch.
Police say a person was punched and has been taken to hospital.
A source said a security guard suffered severe facial injuries in the incident.
St John was also called to the scene.
The incident follows two serious incidents where staff have been injured in the past two weeks.
A nurse was badly burned when a patient poured hot water on her almost two weeks ago.
On Sunday, another nurse was stabbed with what is believed to have been surgical scissors.
WorkSafe is carrying out an assessment of the facility.