More than £11m has been spent covering staffing gaps at the troubled Muckamore Abbey Hospital in the last three years.
The bill is currently running at close to £30,000 a day, figures obtained by this newspaper reveal.
Between 2017 and September this year, agency staff costs totalled £11,575,121.
The six months to September alone saw just over £4.9m spent - £27,040 a day - while in 2019/20 the bill reached £4.7m.
In 2018/19 the bill was just over £1.8m.
More than 60 staff at the facility have been suspended since allegations were made of patients being abused. A PSNI investigation is under way.
Agency staffing costs for the previous financial year - when the first staff suspensions took place - was around £121k.
The costs have been described as "deeply worrying".
Muckamore, in South Antrim, is an assessment and treatment facility for adults with severe learning disabilities and mental health problems.
The Belfast Trust, which released the figures, said the costs include covering suspended staff members but also other gaps.
It has emerged that agency staff, mostly nurses, have been travelling from England and further afield to work at the hospital.
Last week this newspaper revealed how more than £1.5million has been paid to 61 suspended staff.
The employees concerned have been off work and earning wages, in some cases for up to two years.
Seventeen staff suspended for between 18 and 24 months were paid £381,586, Health Minister Robin Swann revealed.
Fifteen staff members have been on "precautionary suspension" for up to six months, nine for between six and 12, 20 between 12 and 18, and 17 between 18 and 24.
Three have been permitted to return to work at the hospital operated by the Belfast Trust.
Fourteen people have been arrested by police probing allegations of abuse, both physical and mental, at Muckamore. No-one has been charged.
Mr Swann has announced that a public inquiry will be held into Muckamore in relation to the abuse allegations.
Concern has been raised at the cost of providing agency staff to Muckamore.
People Before Profit MLA Gerry Carroll, who is a Stormont health committee member, said: "The fact that £11m of public money has been spent on agency staff at any healthcare setting is deeply worrying. But it is especially so when there are so many questions about the level of care provided at Muckamore hospital."
Mr Carroll continued: "These are serious issues that need to be addressed and the Department and Trusts should be upfront with the families of loved ones who have expressed concern about the failure and level of care at Muckamore."
Alliance MLA Paula Bradshaw, who also sits on the health committee, voiced frustration that the Executive has not taken enough action to improve the situation at Muckamore and across the wider system.
"We have raised the issues of agency staff even in an Assembly question this week, and are frustrated we are not much further forward with the workforce reconfiguration proposals outlined in 'Changing the Model of Care' arising from Bengoa (report) which would be the crucial step to fixing this issue at Muckamore and elsewhere," she said.
She added the delay in tackling the issue had been exacerbated by the most recent collapse of power-sharing.
"Here we see yet again the first victims of political gridlock between the two largest parties are the most vulnerable in our society, as this should have been dealt with long ago, but was delayed by the three-year hiatus."
The Belfast Trust said: "The Trust would stress that the agency costs incurred in Muckamore Abbey Hospital are not indicative of the costs of covering suspended staff. Agency costs also include the costs of covering ongoing vacancy, sickness and other absence."
An inquiry into Muckamore was announced by the Health Minister in September, following a damning review of leadership and governance of the hospital.
On Friday it emerged that current and former patients at Muckamore and their families are being invited to help set the terms of reference for the inquiry. Mr Swann said arrangements would be made for those who did not want to appear in public to provide their views in writing or on a one-to-one basis.
Mr Swann said he was "both appalled and saddened that patients and families have been so badly let down by Muckamore Abbey Hospital and understandably their trust in our Health and Social Care system has been eroded".