Six ambulances were turned away from Auckland Hospital on Monday night because its emergency department was too full.
The hospital was under so much pressure patients had to be treated in a public space normally used as back up in case of mass injuries such as from a plane crash or earthquake.
Te Whatu Ora Auckland interim director Dr Mike Shepherd said it was rare to have to divert ambulances.
The situation was "really unusual" in terms of its severity, he said.
Those who could not go to Auckland were sent to North Shore and Middlemore Hospitals instead.
Patients who needed care that could only be offered at Auckland Hospital were still accepted, he said.
"It's not the experience that we're hoping to provide the public," Shepherd said.
An emergency specialist at the hospital and member of the Association of Salaried Medical Specialists, Amanda Rosenberg, said there was no physical space left in the ED on Monday, even every bit of corridor space was being used.
The overflow area that was opened was in the atrium of the main building and was normally a public recreational space but was instead used for ambulance patients still on the stretchers they came in on.
There was no privacy, she said.
"They're already quite frightened that they've had to call an ambulance to come to hospital I would imagine. And then being shunted into a space that's not really designed to care for patients on a regular basis must be even more frightening," she said.
Many waited several hours to get into the department and the ambulance officers had to stay with them, meaning they could not attend other emergencies, she said.
The situation in the hospital ED on Monday had been building for a while.
The hospital was so full, patients who needed a ward bed could not be admitted straight away and took up space in the emergency department.
Some were waiting there two and a half days, she said.
One day last week there were only four beds in the department that could be used for emergency patients, she said.
Australasian College for Emergency Medicine New Zealand chair Dr Kate Allan said the situation was not unique to Auckland - there were extremely full hospitals all over the country.
With this level of pressure already, she and her colleagues worried about what the winter peak would bring.
"It's not new and those working in our emergency departments are reporting that our current levels of overcrowding and prolonged stays in our emergency departments - which is the cause of this - is the worst they've seen."
Overcrowding of EDs had been worsening in recent years, she said, which was a sign of the entire healthcare system being overwhelmed.
"I think this is a symptom of a much bigger disease; it's a symptom of our broken and under-resourced health system."
Patients who were not able to access primary care in a timely manner were more likely to see a deterioration in their chronic conditions and end up presenting at an emergency department, she said.
The fact those patients' treatments had been delayed meant many would also end up in hospital for longer, adding to the pressure on the system, she added.
"There's no doubt that we need a really clear plan for winter as to how we manage because it will be busy."
She told RNZ short-term solutions to the issue would require government, hospitals, specialists within the hospitals and those in the primary care sector to work together, but she was not confident those plans were in place.
"In all honestly no, I'm not confident because we don't know what those solutions are."
But Shepherd said Te Whatu Ora was looking at a range of contributions from healthcare professionals across the workforce to help manage pressures in the system.
"I think we've learnt a lot through previous winters and through Covid and there's a whole range of actions that we are carrying out to try and make the experience and our services better," he said.
"There's a national plan for flow that we're all cognisant of and working on."
He said there was a focus on improving access to primary care and on talking to the public about seeking that care early.
Talking to people about Covid-19, influenza, measles and pertussis vaccinations would also be part of the focus, he said, and pharmacists could be engaged to help reduce some of the pressure on other parts of the system.
"I think we need to look at all of our workforces and understand how they can contribute to the system."
Addressing workforce shortages was the longer-term solution and progress was being made on that front, he said.
"We are looking to recruit as much as we possibly can, we've seen some positives around nursing recruitment."
But he acknowledged the pressures healthcare workers were currently facing.
"It's incredibly hard to be working in the system at the moment."
Shepherd said the Auckland ED was now returning to normal levels.
He wanted to assure anyone who needed care that they would get it.
Operations have been postponed at Dunedin Hospital because of staff shortages and a busy emergency department, it emerged yesterday.
The hospital warned about how busy it was, and apologised to patients who had to wait for care.
There were large numbers of people coming to the emergency department as well as staff illness and vacancies, it said.