OVERTIME payments to prison officers have topped £9.4 million during the last three financial years.
New figures show huge sums are being spent every month to bolster staffing levels at the region's four main jails.
Critics claim the money would be better spent recruiting additional staff, but the Northern Ireland Prison Service (NIPS) has argued it is necessary to meet the demands of a 24/7 operation.
The highest overtime payments were made by bosses at HMP Maghaberry, which houses some of the most dangerous criminals and has segregated wings for paramilitary inmates.
Between April 2012 and March 2015, some £6.15 million was spent on overtime at the high-security jail in Co Antrim which employs 623 prison officers.
During the same period, a further £1.7 million was paid out in overtime at Magilligan Prison in Co Derry, which has 276 prison officers.
At Hydebank Wood in south Belfast, where young offenders and female prisoners are locked up and 175 prison officers work, overtime payments totalled more than £1.5 million.
Finlay Spratt, chairman of the Prison Officers' Association, which represents rank-and-file officers, said: "They should be employing the right number of staff rather than relying on overtime because if you rely on overtime, then you get burn-out.
"But this is the way the service is being run."
Mr Finlay said the prison service was operating with at least 100 officers short and turnover among new recruits was high.
Hundreds of experienced officers have also left through a voluntary redundancy scheme in recent years.
Last year inspectors branded Maghaberry the most dangerous prison in the UK, describing conditions as "Dickensian" in a damning report.
A follow-up inspection found the unsafe and unstable regime had stabilised, but still fell a long way short of required safety standards.
The figures were provided following a Freedom of Information request from the Press Association.
DUP MLA Edwin Poots, who sits on Stormont's justice scrutiny committee, said the level of overtime spend was unsurprising.
He said: "Unfortunately I am not at all surprised by the figures.
"All the indications are that the service has been too reliant on overtime, largely down to the fact that they did not recruit - despite requests from two prison governors at Maghaberry.
"It seems that the prison service headquarters have refused or failed to go and recruit staff to do the job."
A prison service spokesman said: "There are times when overtime is required to meet the operational need of prisons and it is an aspect of running a 24/7 service. Overtime provides a degree of flexibility that can be used to ensure that effective regime is provided for prisoners and to cover unpredictable pressures.
"NIPS has regularly been recruiting since 2012 with job opportunities in the service advertised in 2012, 2013 and 2015. Since March 2015, NIPS has run external recruitment campaigns for prisoner custody officers, night custody officers and custody prison officers. Recruitment interviews are ongoing for all these opportunities with new recruits to begin training in early April 2016."
Meanwhile, figures also show that thousands of days have been lost because officers have taken time off due to stress, anxiety and, or depression.
In Maghaberry last year, 7,919 days were taken off - a significant jump on the 4,321 days lost during 2013/14.
Some 64 prison officers are currently sick leave at Maghaberry, according to the statistics.
Mr Finlay said morale was low.
"The mood is not good, especially when you consider one of our colleagues was blown up just the other week. That trades a lot of fear and there is no point saying that we are big, brave and macho because you can't be brave when someone places a bomb under your car.
"Also, in Magilligan, for example, there used to be four prison officers for 50 prisoners. That has been cut to two members of staff. And, to be honest those fellas and girls are scared. These people have not been put in prison because they missed Sunday school - they are criminals.
"So, there is a big fear factor and I think a lot of the sickness is stress induced by fear because numbers have been cut."
