An earlier version of this story included the incorrect information that being in Code Black meant the ED was unable to accept more patients. Health New Zealand has also clarified that the patient transfer had nothing to do with staffing at Bay of Islands Hospital.

Whangārei Hospital's emergency department hit "Code Black" today after reaching more than 150 percent capacity.

A Health NZ spokesperson said it was not uncommon for the ED to go into Code Black, but it would continue to take patients who needed emergency care.

Health New Zealand Te Whatu Ora Group Director of Operations Alex Pimm said Whangārei Hospital continues to accept patients and new admissions and any claims otherwise are completely inaccurate.

"Patients and their families can be assured that they will be seen and anyone who requires hospital level care will receive it.

"The emergency department uses the Emergency Department at A Glance (EDAAG) system, which has four escalations (green-amber-red-black), factoring in patient numbers, acuity, patient waiting time, and resuscitation bed capacity.

"Colour coding systems are blunt tools used in ED, which is a highly dynamic environment, and the status can change back and forth in minutes over the course of a 24-hour period. It is a real time measure.

"The ED may arbitrarily trigger a particular colour status multiple times in one day as demand and pressures change."

Staff have told RNZ there have been more than 80 arrivals since midnight, including 21 within one hour.

"I have never seen this before," one said.

This afternoon the ED was at 156 percent capacity.

Whangārei's ED - which has space to treat 32 patients - had 50 patients.

Staff said they were being asked to pick up workload from smaller rural hospitals, which were struggling with their own staff shortages.

A medical registrar had been asked to take calls for Dargaville patients and up to nine patients had been transferred from Whangārei, they said.

Bay of Islands Hospital at Kawakawa has been forced to downgrade services this weekend due to a doctor shortage.

However, Health New Zealand said the patients who were transferred from Bay of Islands to Whangārei would have been transferred anyway "due to their acuity and needing to be treated in a larger hospital", and it had nothing to do with staffing at Bay of Islands.

Bay of Islands Hospital at Kawakawa has been forced to downgrade services this weekend due to a doctor shortage.

RNZ understands up to nine patients from Bay of Islands were transferred to Whangārei yesterday, and a medical registrar had been asked to take calls for Dargaville patients, too.

The declaration of a "code black" is made when the situation in a hospital is deemed critical, triggering an emergency escalation response by management.

It is not always restricted to the emergency department, but could involve "bed block" within the hospital itself, which can mean patients who need to be admitted have to stay in ED until beds become available.

At some hospitals, it can mean procedures and operations have to be delayed. However, there is no suggestion that is likely to happen in this instance.

Patients will never be turned away from an emergency department, regardless of the code a hospital is in.

Health Minister Shane Reti has declined to comment.

By Ruth Hill and Kate Green of RNZ