Counties Manukau District Health Board was forced to divert ambulances to Auckland City Hospital after 16 nurses called in sick just before an evening shift this week.
But the board has denied suggestions it considered asking the military to assist in order to keep patients safe, as it grapples with serious staff shortages.
An emerging nursing crisis at Auckland's Middlemore Hospital is getting so bad the New Zealand Defence Force may be needed to assist, a nurses union claimed tonight.
In a statement, New Zealand Nurses Organisation (NZNO) organiser Justine Sachs said ambulances to Middlemore were diverted to Auckland City Hospital on Tuesday due to a lack of capacity, including lack of beds, understaffing and the emergency department being under pressure.
Counties Manukau District Health Board had requested nurses from Auckland District Health Board to provide emergency relief, she said.
However, her statement also claimed it was confirmed to NZNO that Auckland City Hospital was at full capacity so it was suggested that Counties Manukau DHB request emergency assistance from the New Zealand Defence Force instead.
Sachs said this should send alarm bells to those holding the purse strings and who are responsible for resource and workforce planning.
"Nurses are at breaking point," Sachs said.
"Many are telling us they are leaving the profession or going to work overseas because of intolerable workloads.
"Delays caused by diverting ambulances to other hospitals puts patients at serious risk and puts pressure on other hospitals also struggling to cope because they are under-staffed and under-resourced."
Increased demand on the public health system should be expected during the winter season and contingency plans including options for admitting patients to other hospitals and sufficient staffing should be in place, Sachs said.
"Given the enormous stress on our under-resourced health system it's frightening to contemplate how hospitals in the Auckland region would cope should there be a significant catastrophic event."
A Counties Manukau DHB spokesperson said "under no circumstances" had the board considered requesting assistance from NZDF.
"The health and wellbeing of patients is our number one priority."
But it confirmed 17 patients were diverted to Auckland on Monday night after 16 nurses called in sick.
"We can confirm that ambulances were temporarily diverted to Auckland District Health Board ... for two hours from 7.30pm-9.30pm due to the increased number of presentations to the Emergency Department at Middlemore Hospital.
"This was necessary as 16 nurses called in sick just before the beginning of the evening shift.
"The decision to divert was to ensure that patients who were acutely unwell could be seen within appropriate timeframes. As a result, 17 patients were diverted, of which six were admitted to Auckland City Hospital."
The spokesperson said the diversions also enabled patients already at the ED to be seen and allowed Middlemore Hospital to bring in more nursing staff to support the opening of additional beds for patients requiring admission.
An Auckland DHB spokesperson also dismissed suggestions it had suggested bringing in military personnel.
"These claims are not accurate. We are not aware of anyone from Auckland DHB leadership suggesting that Counties Manukau DHB request emergency assistance from the New Zealand Defence Force.
"It is also inaccurate to say that Auckland City Hospital is currently at full capacity."
NZME has sought comment from NZDF.