The A&E department at Craigavon Area Hospital had one of its "worst ever evenings" on Wednesday.
It comes as as non-emergency operations were suspended at a number of Northern Ireland hospitals due to staff shortages.
Craigavon hospital's medical director Dr Maria O'Kane spoke of the strain and "relentless pressure" staff were faced with and how 83 patients waited more than 12 hours for treatment on Tuesday evening.
“Unfortunately, there has been very little respite this year in terms of the activity across the different seasons. This pressure is relentless, day on day," she told UTV.
Her comments come after a medic at Antrim Area Hospital addressed a packed waiting room warning of lengthly waits for treatment.
Dr Hugo Dowd made headlines after he said the backlog of patients waiting for a hospital bed was slowing down the speed at which his staff could treat people arriving at the ED.
Alliance councillor Eoin Tennyson said the wait for patients was "utterly appalling and totally unacceptable".
“It has been clear for some time now that our hospitals are overloaded and fast approaching breaking point. We desperately need to proceed with the transformation of our health and social care system to improve access for patients and alleviate pressure on A&E departments." he said.
“We also must act, as a matter of urgency, to address pay and workforce planning issues in the sector."
Northern Ireland's health service is coming under increasing pressure after the Royal College of Nursing and Unison health staff voted in favour of striking after a pay dispute and concerns over patient safety.
RCN members are set to strike on December 18, with Unison set to take part in their first phase of industrial action short of a strike from November 25 to December 18.
Non-emergency operations have been suspended at a number of Northern Ireland hospitals due to staff shortages.
The BBC has reported operating theatres are not able to work at full capacity due to the shortages of staff, particularly of skilled theatre nurses.
The Belfast, Southern and Western Trusts are affected by the issue. Emergency surgery will go ahead as normal.
A Belfast Trust spokesperson said it could not say how long the restrictions to surgeries would remain in place.
The Southern Trust confirmed it had cancelled a number of surgeries in Craigavon and in Daisy Hill.
"Our priority is to ensure that there are appropriate numbers of nurses with the appropriate level of skill in our theatres to keep our patients safe," a spokesperson said.
"We have been left with no option but to temporarily suspend some surgical sessions. Emergency, red flag and urgent surgery is continuing as normal."
The Western Trust said it was experiencing similar issues.
"The trust is currently experiencing issues with theatre staffing shortages in Altnagelvin hospital and as a result not all theatres are running at capacity," a spokesperson said.
"In order to ensure the number of staff remain within agreed safe levels, an average of ten theatre sessions per week are currently not providing their routine service.
"Emergency theatres, trauma theatre and labour ward are not affected. The Trust has an ongoing recruitment process in place to improve staffing levels in theatres."