One Northern Ireland emergency department saw a record 332 patients in a single day - long before the usual pressures of the festive season, it can be revealed.
The Ulster Hospital in Dundonald saw an average of one new patient roughly every four minutes on October 23.
And in November, the Ulster's A&E saw a 6.8% increase in demand compared with the same period the previous year.
The South Eastern Trust said that its emergency departments, particularly at the Ulster, had been under "significant pressure" over the festive period due to an "increase in attendances" and the "complexity of illnesses and patients treated".
Yesterday, the trust issued an appeal on Twitter for off-duty staff to come in to work last night - the second such appeal since January 1. On New Year's Day it tweeted a warning that its emergency departments and hospitals continued to be "extremely busy". However, in contrast to the Northern Trust, which was forced to bring in four St John Ambulance volunteers to work at Antrim Area Hospital on New Year's Night, the South Eastern Trust said it did not use the voluntary organisation to assist on hospital wards during winter pressures.
Last Wednesday and Friday the Northern Trust tweeted to advise the public that A&E at Antrim Area Hospital and the Causeway Hospital in Coleraine were "extremely busy" and asked people only to attend if they were "very seriously ill or injured" and required urgent treatment.
Then, on New Year's Day, it launched a Twitter appeal for nursing staff and healthcare assistants to volunteer to work, revealing that Antrim Area Hospital and the Causeway Hospital were facing "severe pressure".
The Northern Trust said volunteers from St John Ambulance had "provided some input at different times at Antrim Area Hospital over the two nights of New Year's Eve and New Year's Day, working under our volunteer arrangements".
It said: "On the night of New Year's Day, four St John Ambulance volunteers worked at Antrim Area Hospital.
"This complemented our normal service and assisted in supporting patient comfort and safety.
"The volunteers worked under the direction and delegation of registered nurses, and the additional support provided helped to alleviate the pressures on very busy staff, thus allowing them to focus on other priorities.
"Over the Christmas and New Year period we also used the services of both bank and agency nursing staff, but, as will be appreciated, these resources were extremely stretched over the holiday period.
"We do not have a contract with St John Ambulance."
Meanwhile, the Belfast Trust said its emergency departments had also been under pressure over the Christmas period due to an increase in attendance. However, staff were not called in from leave.
A spokesperson said: "We had contingencies in place to help alleviate the additional pressures we traditionally experience at this time of year, and we pay tribute to our staff who have gone the extra mile over this busy period.
"Belfast Trust staff were not called off leave as contingencies were in place as part of our winter pressures planning.
"The Belfast Trust do not use St John Ambulance to assist on hospital wards during winter pressures as contingencies were in place as part of our winter pressures planning."
The Southern Trust described its emergency departments as "very busy over the Christmas and New Year period".
It said that staff had "once again gone the extra mile in responding to the increased demand for services, with additional staffing available as required to cope with the high volume of patients."
It added that voluntary organisations "do not work on hospital wards in the Southern Trust".
A spokesperson for the Western Trust said they were unable to respond to a request for information.
Former DUP health minister Jim Wells said New Year's Day and January 2 were traditionally among the busiest days for the health service.
"Over Christmas you have doctors unavailable, and on the first or second day back after the holidays you have huge pent-up demand for services," he said. "People can't book appointments, we are really short of GPs, and they turn up at hospitals.
"We haven't the staff and there is increasing demand, with the budget having to be stretched further and further.
"Long term, the only solution is to implement the reforms recommended in the Bengoa, Compton and Donaldson reports."
Former UUP health minister Michael McGimpsey called the situation "deplorable" and said it "could have been avoided" if more support had been made available to hospital and ambulance staff.