Auditors have revealed that a contract to supply lung patients with oxygen cylinders in their homes cost three times more than originally predicted.
A report found that the company providing the cylinders struggled to keep up from the beginning, partly because of "inadequate" information from the NHS on the likely demand for the service.
Supplying oxygen at home under the contract is costing about £6m a year, compared to the original estimate of £2m.
In February 2006, a five-year contract with Air Products replaced the old system where patients got cylinders from the local chemist with a prescription from their GP.
The recently released Wales Audit Office report now says not all the intended benefits of the move have been realised.
It says the time set aside for preparing for the contract was very tight and did not allow for a back-up plan should things go wrong.
The Auditor General for Wales, Jeremy Colman, said: "The change in the arrangements for home oxygen services was not well managed and the reasons why those arrangements failed were foreseeable.
"It is disappointing that some of the intended benefits to patients have yet to emerge.
"It is even more disappointing that previous lessons around contingency planning, risk management and communication with those affected or responsible for delivering change were not applied in this case."