Police have been called to to a serious assault at Christchurch's Hillmorton Hospital about 9.30am this morning.
Police say a person was punched and they were transported to hospital.
According to a staff member, a security guard received severe facial injuries in the altercation.
St John has confirmed it was called to the scene.
It follows two serious assaults against nurses at the mental health unit in the last two weeks
Worksafe is carrying out an assessment of the facility.