The cost of dealing with the shortage of doctors at Daisy Hill Hospital's emergency department has increased more than fivefold in three years.
In 2014 the amount paid out was just more than £500,000. Last year that figure jumped to almost £3m.
The Southern Health Trust has NI's highest number of doctor vacancies.
A medical recruitment agency is currently advertising 64 vacancies in the Southern Trust area, which includes Daisy Hill and Craigavon Area Hospital.
Across Northern Ireland's five health trusts, there are 165 vacant positions.
In an unusual move and in order to cope with the staffing issue, it has emerged that the Southern Health Trust is due to advertise for a permanent joint consultant appointment between Daisy Hill and the Belfast Trust.
In May, the Southern Health Trust revealed just how problematic it has been keeping Daisy Hill's emergency department open.
Last year the bill for hiring locum - or temporary - doctors in Daisy Hill was almost £3m. Craigavon, the trust's main hospital, spent £1.9m.
The Southern Health Trust said it was relying too heavily on locum doctors to fill shifts in the emergency department and threatened to suspend its overnight service.
After protests against the potential closure of the emergency department, the Department Of Health announced a temporary reprieve in May.
At a regional summit with representatives from the Ambulance Service, the Belfast Health and Social Care Trust and the Public Health Agency, it was agreed that Daisy Hill's emergency department would remain open, using doctors from the Belfast Health Trust when necessary.
But a Southern Health Trust spokesperson told BBC News NI it hasn't had to use doctors from Belfast and is still relying mostly on locums.
The Southern Health Trust isn't the only trust relying on locums: In Fermanagh, more than £2m was spent staffing the South West Acute Hospital; the emergency department at Altnagelvin, Co Londonderry, spent more than £1m.
The Royal Victoria Hospital in Belfast, which has the biggest emergency department in Northern Ireland, spent about £2.3m bringing the overall cost across the five health trusts over three years to almost £20m.
But the problem isn't going away. According to medical recruitment agency Medical Banks' website, 165 locum doctors are urgently required.
The figure highlights that workforce planning remains a serious issue for the Health Service.
A majority of the vacant positions - 64 in total - are in the Down area. Thirty one are in Derry, 26 in Fermanagh and five in Belfast.
The shortage of expert medical staff means doctors can command high rates. At short notice some trusts are offering as much as £600 per shift - with reports that some weekend shifts are carrying a tag of more than £1,000.
Figures obtained by the BBC in a Freedom of Information request also show the highest individual payments made to doctors doing shifts in emergency departments.
The Northern Trust's Causeway Hospital comes in the highest, on one occasion paying a locum consultant £172 per hour. The Royal Victoria Hospital in Belfast has paid out more than £150 per hour.
Previously, the Southern Health Trust has said that despite numerous attempts to recruit at Daisy Hill, doctors appear to want to work in bigger hospitals such as Craigavon and the Royal Victoria Hospital.
An ongoing national shortage of doctors is also contributing to the problem.