Only one of the field hospitals set up to help ease demand during the coronavirus pandemic has treated any patients, despite costing £166m.
Buildings, including sports stadiums and theatres, were rapidly turned into 17 hospitals at the peak of the crisis.
But only the Dragon's Heart Hospital in Cardiff has received any patients.
Health Minister Vaughan Gething defended the decision saying in hindsight he would have "made different choices at the time".
"If we had needed extra capacity, in much greater numbers, and I had not acted as I did, I think the public would have rightly said, 'Why on earth didn't that man in charge do something about it?'," he said.
In a bid to ease pressure on hospitals, 17 field hospitals were rapidly built to provide 6,000 additional beds, doubling the Welsh NHS's capacity at the peak of the pandemic.
It took just eight weeks to build those field hospitals, and two additional community step-down facilities for non-coronavirus patients.
Since the Dragon's Heart Hospital at the Principality Stadium opened in April, 46 patients have been treated at the home of the Welsh Rugby Team, which is now empty and on standby in case of a second wave.
No-one has been admitted to the other 16 locations around the country, including the 1,000-bed facility at Swansea Bay Studios and Venue Cymru in Llandudno, Conwy.
Mr Gething said the Welsh Government had set up the hospitals after seeing how parts of the health service in Italy had been overwhelmed.
A review is due later this month to decide how many field hospitals will be retained or repurposed.
A health economist from Swansea University said that, in hindsight, spending £166m on 46 patients was "not a good use of limited resources".
Prof Ceri Phillips said: "Elective operations were cancelled, the level of demand was reduced, out-patients was reorganised so people weren't coming into hospital."
Mr Gething said it was possible "fewer field hospitals" would be needed in the coming months.
"It's also possible we can make more use of the current capacity we have as we restart more normal NHS activity," he added.
The owner of Swansea Bay Studios, Roy Thomas, signed a lease allowing the Welsh NHS to use the largest building on his site free of charge for a year to help tackle the virus.
But Mr Thomas said the complex needed financial support to survive.
He said: "I see myself in next 12 months serving two masters.
"One is the health authority, to maintain whatever requirement they have 12 months in the future. and the other is to attempt to play my part in maintaining the filming industry for Neath Port Talbot, Swansea and west Wales."
Swansea Bay UHB's Chief Operating Officer, Chris White, said it was fortunate the beds at the two field hospitals had not been needed at the peak of the pandemic.
But while cases have fallen, Mr White said it was important the field hospitals were "ready in case we need them".
Venue Cymru in Llandudno also faces financial challenges after giving over its building to create 350 extra beds for the Betsi Cadwaladr health board.
The theatre and conference centre, which is owned by Conwy council, has a three-month contract to act as field hospital until 6 July.
The deal, which is based on it having no financial impact on the local authority, will then be extended by agreement on a month-by-month basis.
But Venue Cymru said it would need financial help to restart shows after the pandemic.
Mark Wilkinson, from Betsi Cadwaladr health board, said the focus was now on how to "resume services which had been put on hold to manage the initial wave of Covid-19 patients".
Bangor University agreed to a three-month minimum lease with the Welsh Government for the use of the Canolfan Brailsford sport centre at Bangor University, to be reviewed on a monthly basis after that.
The Welsh Government review into field hospitals will look at how many extra beds the Welsh NHS might need if the transmission rate of coronavirus begins to rise towards a second peak.
It will also examine whether the field hospitals can be used to help the resumption of medical services that were suspended while the NHS focussed on treating Covid-19 cases.
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Cardiff and Vale
The health board has said the Dragon's Heart Hospital site would be "retained until the autumn and will be available to admit coronavirus patients should a surge in capacity be required".
The Welsh Rugby Union, who own the stadium, have said the contract for it to remain as a field hospital has been extended until early autumn.
There are three field hospitals, Deeside Leisure Centre in Queensferry, Canolfan Brailsford sports hall at Bangor University and Venue Cymru in Llandudno.
The three "Rainbow hospitals" are "available and ready" to accept patients but the health board said the focus is now to "resume services that were put on hold to manage the initial wave of Covid-19 patients".
There are nine field hospitals here: Bluestone Wales resort in Pembrokeshire; the leisure centres in Cardigan, Carmarthen, Llanelli and Plas Crug (Aberystwyth); two at the Parc y Scarlets in Llanelli, one in the stadium and the other in the training 'barn'; the Selwyn Samuel Centre in Llanelli and Ysgol Penweddig in Aberystwyth.
Hywel Dda health board said it was "exploring many different options" for the next 12 months including "different uses of our facilities" as part of national discussions.
Cwm Taf Morgannwg
Bridgend Industrial Estate is the only remaining field hospital in the region after it was announced the 290-bed hospital at the WRU's Centre of Excellence at Hensol was being decommissioned this week.
The health board is also operating "community step-down facilities" Abergarw Manor in Bridgend and Marsh House in Merthyr Tydfil for non Covid-19 patients to move out of hospitals.
Cwm Taf health board said field hospitals were "essential to their planning" so that extra capacity is in place "should we need it to respond to the challenge of Covid-19".
There are two field hospitals in the area, one at Llandarcy Academy of Sport (where the Ospreys rugby team usually train) and one at Swansea Bay Studios.
Swansea Bay UHB's Chief Operating Officer Chris White said: "Flexibility is an important factor in the management of the situation as we move into the next stages."
There are no official field hospitals here, although the health board has said the new Grange University Hospital in Cwmbran is "ready if we need to use it."