Nurses are jetting out of Northern Ireland to work in England at weekends where they can earn triple the hourly rate here - up to £600 for two shifts.
They fly over on Friday nights and work Saturday and Sunday shifts for private agencies in English hospitals, before returning to Belfast wards on Monday.
The Royal College of Nursing in Northern Ireland said that if nurses in the province were being paid properly they would not have to take on the extra work.
There are currently around 1,500 nursing vacancies in Northern Ireland.
The Belfast Health Trust confirmed that some of its staff have been deciding to work for a private sector health care company.
They supplement their income by working for the private agencies, which then charge the National Health Service.
Despite concern over the revelations the trust insisted the additional shifts are not putting a strain on the province's already-stretched health service.
It emerged yesterday that after a full week's work, the Belfast nurses catch a Friday evening flight to London, Birmingham or Tyneside, where they provide weekend cover.
Despite working the Saturday and Sunday shifts and travelling hundreds of miles, the tired nurses then start work as usual in Belfast on Monday.
A Sunday newspaper report yesterday claimed each nurse is being paid up to £600 for two shifts.
But the extra costs, which include airfares, taxis and accommodation, take the total cost to the taxpayer to well over £1,000.
It is understood a group of 10 to 12 nurses from the Royal Victoria Hospital and Belfast City Hospital have been making the 480-mile round trip to work at Sunderland Royal Hospital.
Janice Smyth, director of the Royal College of Nursing in Northern Ireland, said: "They are choosing to use their days off to go and do this and we are talking about highly trained nurses."
She added: "I was not aware of this, but I am not surprised. I don't know any of these nurses personally, but nurses are really struggling financially. We know that a high percentage of them are struggling to pay their gas and electricity bills.
"There is a global shortage in nursing at the moment, but the problem is particularly acute in the United Kingdom."
Mrs Smyth said nurses in the province have not had a full pay increase since 2011. "We are now in 2016 and the Department of Health has still not made any offer," she added.
The RCN chief said there are 16,500 nurses in Northern Ireland at present, but there are around 1,500 vacancies.
The Belfast Health and Social Care Trust said: "The trust is aware of an arrangement whereby a number of its employees also choose to work for a private sector healthcare provider.
"The trust does not believe that this has an impact on the service provided by the trust as this private arrangement is outside of the staff's contractual obligation to the organisation."
The TaxPayers' Alliance said the public would be staggered at the sums spent flying nurses around the UK.
Jonathan Isaby, chief executive of the organisation, said: "Expensive agency staff should only be used by NHS trusts when absolutely necessary, not as a matter of course.
"This represents a wasteful allocation of resources."
The costs were said to be met by recruitment agency Medinet, a business based in Wales which describes itself as "the leading company working alongside the NHS to guarantee that waiting time targets are achieved".
Medinet operations manager James Warnock said: "Medinet was awarded the contract to support endoscopy services at Sunderland Royal Hospital following a competitive tender process.
"All travel costs are covered by the company within the cost of service wherever the nurses travel from."