Overcrowding in emergency departments has sparked a "dangerous" trend of paramedics being told to keep patients waiting in ambulances for up to two hours, a top emergency doctor says.
Experts warn it's putting patients at risk and ties up ambulances needed for other callouts.
The Health Minister confirmed to the Herald he was aware of the problem at some of the country's busiest hospitals, and blamed it on underfunding by the previous government.
St John said, while significant delays do happen, it had processes in place to minimise impacts - such as diverting non-urgent patients to medical clinics.
The Herald understands the practice - dubbed "prolonged offloading" or "ambulance ramping" in Australia - is happening daily across the country and has been getting progressively worse in the past five years because EDs are at "crisis point".
John Bonning, Australasian College for Emergency Medicine (ACEM) president and Waikato Hospital emergency doctor, told the Herald it was now common practice in New Zealand for hospitals to contact an ambulance en route to the hospital to request it goes "offline" for up to two hours while they clear space in ED.
"It's dangerous as it takes an ambulance crew offline so they are not available for other callouts. So there is risk to other patients as well as the people being kept in the ambulance," Bonning said.
"This is a direct consequence of the growing demand on EDs."
It wasn't happening to critical patients, he said, and no one was dying in a stretcher, but frail elderly patients were being affected by this frequently, and they were suffering worse outcomes as a result.
St John medical director Dr Tony Smith said his organisation did not experience ramping "in the same way as our counterparts in Australia and the UK face" but added "emergency department overcrowding is a significant issue throughout New Zealand, which affects patients, families, the emergency ambulance service and the wider health system".
"We also acknowledge there are times when the pressures faced by District Health Boards have a flow on effect to our ambulance services. St John works closely with DHBs to limit the extent of these issues," he said.
"St John has established processes, including an escalation policy, whereby managers are deployed to work with clinical leads at emergency departments to optimise the process of transferring the care of patients from an ambulance to emergency departments.
"We also prioritise transporting patients to medical centres and other healthcare providers, if feasible and safe. Additionally, we have a national system (Clinical Hub) whereby we assess patients over the phone and advise them of alternative treatment pathways that limit the number of unnecessary presentations to the emergency department."
He said "while there are occasions when our ambulance officers experience significant delays with transferring patients into the care of emergency departments, particularly during winter, as a principle, St John does not support ramping".
New Zealand Resident Doctors' Association national secretary Deborah Powell, who spoke to the Herald on behalf of the union's members, said it was a huge problem - and another example of how overstretched hospitals are.
Overcrowding in EDs was a flow on effect from the lack of beds available in the wards, she said.
Patients were being held in ambulances because there wasn't enough room in EDs, and patients in EDs were being held there because of the delay in discharging patients in the wards, Powell said.
"Patients aren't getting discharged timely because if you have a patient needing urgent care and one needing to be discharged of course you are going to prioritise the one needing urgent care," Powell said.
It comes as Auckland City Hospital last week hit its highest ever number of sick patients treated in one day and last month ambulances at Middlemore Hospital had to be diverted elsewhere when 16 nurses rang in sick.
ACEM called on the Health Minister to commit to urgent action to address the crisis.
More general practices need to operate after hours and at the weekend to stop EDs getting "clogged up" with patients who don't require urgent care, it said.
Health Minister David Clark said he was aware of "ambulance ramping" taking place in New Zealand. He discovered it two years ago in the final months of the last government.
"It is utterly unacceptable that the previous government's health funding was so scant that emergency departments struggled to meet demand from New Zealanders in need," Clark said.
The ministry had taken a number of steps to address the issue, he said, such as making doctor visits cheaper for 600,000 New Zealanders, but pressure wouldn't be relieved straight away.
"The pressure on emergency departments developed over nine years ... we're getting started and making good progress," Clark said.
National's health spokesman Michael Woodhouse hit back, saying it was time the minister stopped blaming the previous government for his own poor management of the health sector.
"We're facing flatlining pharmaceutical funding, longer surgery waiting times and a delayed cancer plan, and that's all down to this government. It's not a surprise there's pressure across the sector," Woodhouse said.
A 2008 report - conducted by a government health advisory group - detailed warnings of ambulance ramping to then health minister Tony Ryall.
"Ambulance ramping is clearly detrimental to ambulance services, particularly as it prevents ambulances responding to new emergency call-outs, thus presenting an additional hazard to the public," the report stated.
Bonning said he was aware of this practice happening at Auckland City Hospital, Waikato Hospital, Canterbury Hospital and Counties Manakau.
The Herald approached each DHB for comment.
A Waikato DHB spokeswoman said she was unable to respond as the hospital was "extremely busy".
A Counties Manukau DHB spokeswoman said it did not support ramping of patients but sometimes ambulances had to wait due to high demand.
Auckland City disputed the claims, saying it does not treat patients in ambulances or ask paramedics to keep treating patients in ambulances.
Canterbury DHB chief medical officer Dr Sue Nightingale said ramping did not occur at Christchurch Hospital's emergency department and Canterbury DHB did not support this practice.