More than £1,600 was paid out for a single agency shift in the South Eastern health trust last year, it has emerged.
Health officials have been urged to introduce a cap on "rip-off" agency fees.
In some cases trusts have spent up to £155 an hour to fill gaps in their workforce.
The cost of hiring temporary staff is currently running at £640,000 a day.
Spending has surged by 160% since 2015, and is on course to hit £230m this year.
Today new details of the huge fees charged for agency staff can be revealed. Documents obtained by this newspaper show:
The Southern Trust and South Eastern Trust pointed out that their spending related to specialist staff drafted in on bank holidays.
However, former Health Minister Edwin Poots said it was difficult to justify the costs. He called for measures to cap the fees agencies can set.
Four years ago the then Health Secretary Jeremy Hunt announced a framework to bring an end to excessive charges. However, a similar cap has never been implemented in Northern Ireland.
That definitely would be desirable to put a stop to this, because the system is being ripped off by agencies who are making extreme profits. Edwin Poots
Mr Poots said: "It is clear that, once the workers have been paid, these companies are taking off large amounts of money in profit."
Mr Poots said an overhaul of the system was needed to cut agency spending, claiming it was "badly mismanaged".
"I don't think the health service has a choice at present - you can't compromise patient safety," he added. "The costs are not justifiable but without agency staff you can't sustain the health service."
The DUP MLA for Lagan Valley, who was Health Minister from 2011 to 2014, said a growing exodus of staff was driving up reliance on agencies. "Some people in full and part-time employment are leaving to go to agencies because they offer better terms," he added. "There is an all-round lack of staff and we need more nurses trained."
Last month it emerged the Belfast Health Trust had spent £4m on agency staff to cover vacancies at the scandal-hit Muckamore Abbey Hospital, where 36 employees have been temporarily suspended pending an investigation. Agency staff are being paid up to £40 per hour, compared to local hourly rates of £9, the BBC reported.
Latest Department of Health figures show there are 7,000 vacancies across the system, including a shortage of 3,000 nurses and midwives.
In the 12 months to April this year the cost of temporary workers totalled £201,298,475 - £550,000 a day on average. The cost has risen year on year, from £76,508,610 in 2014/15.
In the first three months of the current financial year it totalled £58,043,621 - equivalent to £637,842 every day.
If it continues at this rate the 2019/20 bill will pass £232,000,000.
A breakdown of individual trusts' spending on agency staff has been obtained by this newspaper.
The Belfast Trust, Northern Ireland's biggest trust, had the highest bill, running at almost £62m in 2018/19.
It could not provide details of the highest rates it paid.
The South Eastern Trust spent £24.5m in the same 12-month period. Its highest rate was £131 per hour, and £1,640 for a shift. A spokesperson said the figures were for an "off-contract agency on bank holiday rates for 12.5 hour bank shifts in a highly specialised area within the prison regime".
The Northern Trust paid £40.7m. Its highest rate was £141 an hour for a locum consultant.
A spokesperson said it was for a specialist doctor to cover a vulnerable service.
The Western Trust spent £33.8m on agency staff, with a top hourly rate of £155 for a medical locum.
The Southern Trust's bill was £33.6m. Its top hourly rate was £131. A spokesperson said it was for "a specialist registered nurse on a bank holiday".
The Department of Health acknowledged that agency spending must be reduced but did not address if a cap would be introduced. A spokesperson said: "The department is fully aware of the unsustainable nature of this expenditure, particularly at a time of serious financial pressures right across the public sector; this is why transformation of health and social care in Northern Ireland is such a priority.
"The department is currently working with HSC employers on very detailed proposals to reduce all agency and locum spend in Northern Ireland, beginning, as a first step, to reduce and eliminate off-contract agency expenditure.
"We hope to be able to discuss some firm proposals with trade unions shortly.
"In the meantime, the primary aim is to ensure that safe staffing levels and effective services are sustained at all times for patients and clients."