The statistics show the Welsh NHS spent more than £6m on private treatment in the last two years.
Hywel Dda health board spent six times more than any other board - £3,676,211 between April 2013 and March 2015.
It said it had used private providers after facing recruitment problems, to avoid patients suffering longer delays.
Five out of the six health boards in Wales all paid for so-called "spot contracts" during the period - short-term, unplanned contracts awarded to private healthcare providers to carry out NHS work.
Winter pressures, long waiting lists and staff vacancies are all reasons given for why this was necessary.
Figures uncovered using a Freedom of Information Inquiry show Hywel Dda health board paid for 22 spot contracts over the two years, to reach the £3,676,211 total figure for private spending.
Most of the other health boards spent in the region of hundreds of thousands of pounds, but Aneurin Bevan spent nothing.
Welsh Institute for Health and Social Care director Marcus Longley questioned whether the decision to pay the private sector amounted to good value for money.
"In Wales the policy is to do everything in-house within the health service whenever possible.
"The difficulty is that if you go to the private sector in January wanting lots of operations done by March you'll pay through the nose for that."
Hywel Dda health board said difficulties with recruitment had led it to use external providers, but it was continuing to review this.
In a statement the board added: "The decision to allocate additional funding was in recognition of the importance of ensuring our patients received planned treatment, such as hip or knee replacements, when demand for emergency services was significant.
"Without this immediate solution more patients would have waited longer for treatment."
There is a wide variation in how much Welsh health boards spend on private healthcare, figures uncovered by BBC Wales have revealed.