A woman brought to Auckland's North Shore Hospital by ambulance was punched in the face and knocked to the ground by a man she did not know as they waited to get into the overcrowded emergency department.
She had intervened when he abused the sole nurse working in the ambulance bay where they were queuing.
The incident happened on Tuesday night when an overflow of patients were trying to get into the emergency department.
RNZ understands the man had been brought in by police, but they had left him there.
The man began to throw things around the area and was abusing the nurse overseeing patients.
The woman patient, who was sick enough to have been brought in by ambulance, reprimanded the man and he then punched her in the face.
The New Zealand Nurses' Organisation confirmed the incident, saying it had been reported to it and it was investigating.
North Shore Hospital's acting manager of acute and emergency medicine Brett Paradine said security staff attended immediately and de-escalated the situation.
Police were called but the man had left the hospital before they arrived.
Staff, led by nurses, raised fears about the security situation in the emergency department in a formal health and safety complaint to their bosses two weeks ago.
They were being subject to more violence and were dealing with more mental health patients and those affected by drugs and alcohol but there was no increase in security, the complaint said.
Staff also raised concerns about patients having to wait in ambulance bays and the lack of oversight there.
Paradine said the woman was assessed by staff immediately and was still in hospital but for reasons unrelated to the incident.
The police did not respond to any of RNZ's questions, saying only they attending a "disorder" incident and were investigating.