The Northern Ireland Prison Service has spent more than £9.5 million in overtime since 2009, it has been revealed.
The payments, which have risen year-on-year, equate to more than £183,000 a week.
Some prison officers have boosted their annual salary by up to £16,000 from working additional hours, the figures released under Freedom of Information (FOI) legislation show.
Finlay Spratt, of the Prison Officers' Association, said: "These figures clearly demonstrate that the staffing levels across the Northern Ireland Prison Service are far too low. They are paying overtime but, speaking as a trade unionist, they should be cutting out on overtime and recruiting more staff. But it's cheaper to pay the overtime.
"Let's be honest, nobody in the current economic climate is going to say no to a bit of extra cash. They are not objecting to overtime but I, as a trade unionist, am - I think they should be getting more people into employment."
They show that since 2009, staff at the high security Maghaberry jail in Co Antrim, where murdered prison officer David Black was employed, worked the most overtime. A total of £5.6 million was paid out which was almost four times more than at any other facility.
Payments peaked this year when £1.7 million was spent between March and October. Until recently more than 40 dissident republicans at Roe House, Maghaberry, were engaged in a dirty protest which involved mixing urine and excrement and spreading it on cell walls. The protest over strip searches was called off last month.
In its FOI response the Department of Justice said overtime was a normal aspect of running a 24/7 service. A statement said: "This analysis shows that additional hours are worked by staff across NIPS (Northern Ireland Prison Service) but particularly in the three prisons and the Prisoner Escort and Court Custody Service. This approach has been in place over many years and is a normal aspect of running a 24/7 operational business."
Workers at Magilligan prison in Co Londonderry were paid £1.3 million in overtime since March 2009. At Hydebank Wood, which accommodates women and young offenders, the overtime bill reached £1.2 million over the four years. The Prisoner Escort and Court Custody Service generated an overtime bill of £2.7 million during the same period.
SDLP MLA Patsy McGlone, who sits on the justice committee at Stormont, said prison management may need to reassess how resources are deployed. "Having been to Maghaberry on a number of occasions, it never ceases to amaze me how many prison officers it takes to escort a prisoner. I understand some of these guys are high risk but it does puzzle me as to why, as I have seen, it takes five prison officers to escort a prisoner. If overtime is being spent unnecessarily it really has to be looked at again."