The union for nurses says Hawke's Bay Hospital is failing in its duty of care and is demanding change.
It issued a provisional improvement notice to the district health board, saying it legally had to address health and safety concerns by 5 October.
New Zealand Nurses Organisation organiser Sue Wolland said repeated attempts to escalate concerns about dangerous working conditions that threaten patient safety at the emergency department had been ignored or minimised.
"Staff are working under incredible pressure which is severely impacting their physical and mental health.
"The department is well beyond capacity every day which means staff are unable to respond to people needing emergency care in the time required to best ensure their well-being."
Wolland said issues included: patients being housed in inappropriate or hazardous places such as corridors, dangerous delays in triage and assessment, seriously unsafe and inadequate staffing levels, nurses too overworked to take meal and other breaks, patients missing out on essential care, increased risk of error and staff feeling unsafe and anxious while at work.
"ED staff, including those in leadership, have repeatedly raised these concerns with the DHB and minor solutions have been proposed that never seem to eventuate.
"What we have here is a serious or sentinel event just waiting to happen, resulting in avoidable patient death and the potential end to nursing careers.
"These concerns need to be urgently addressed, including contingency plans to cover staff sickness and to ensure staffing meets levels required for safe and acceptable standards of patient care."
Extra staff appointed to help with demand, DHB says
In response, the DHB's chief operating officer Chris Ash said the DHB was "committed to resolving concerns".
He said pressures on the emergency department had been "unrelenting", as the department had faced the increased seasonal demands of acutely sick patients, compounded by the RSV outbreak, subsequent Covid-19 lockdown challenges and wards near or at capacity.
"In recent months the DHB has placed significant investment on the recruitment of senior clinical FTE [full time equivalent staff] with five additional senior medical officers and 10 additional nursing staff to the emergency department. Redesign work to support better patient flow is also in progress.
"We will work through this process in an open, transparent and compassionate manner with all parties."