The use of management consultants is 'commonplace,' said the WLGA
Local authorities in Wales have defended spending at least £16.5m on management consultants.
Conwy Council, the biggest spender last year at £3.1m, said most of it did not impact on council tax payers.
Flintshire spent nearly £2.2m and Merthyr Tydfil spent nearly £2.3m.
Unison said they seemed to be "eating up scarce resources" but the Welsh Local Government Association said councils did not always have in-house expertise to call on.
The figures were obtained by a Freedom of Information (FoI) request by the BBC's Wales at Work programme. Of the 22 councils in Wales, 17 responded within the time limit.
The total amount councils spent on management consultants varied greatly, with Conwy, Flintshire and Wales' smallest council Merthyr Tydfil each spending more on outside advice than the Welsh Assembly Government.
MANAGEMENT CONSULTANT SPENDING
Merthyr Tydfil £2,276,000
Neath Port Talbot £781,457
Caerphilly £675 411
Blaenau Gwent £613,812
Vale of Glamorgan £50,224
Cardiff, Newport and Anglesey No response to FoI requests within time limit
RCT and Pembrokeshire FoI request not met because of cost reasons
Welsh Assembly Government £1,964,365
Spending on management consultants. Source: Freedom of Information request by BBC Wales
On the other end of the scale, Carmarthenshire Council did not engage the services of any external consultancies last year. The local authority said it had not identified a need.
In 2007/2008, the Welsh Assembly Government spent nearly £2m hiring the services of consultancy firms like Pricewaterhouse Coopers, Bravosolutions and Elan.
Paul Elliott, head of local government for Unison in Wales, expressed deep concern about the public sector's use of consultants.
He said: "There's only a certain amount of money available for public services."
Conwy Council said a substantial amount of the council's expenditure was supported by Welsh Assembly Government funding and other grants, and did not directly impact council taxpayers.
A spokesman said: "We don't have spare capacity and therefore when we need specific expert advice on capital schemes or on large flagship projects, we buy in the expertise as and when required."
In a statement the Welsh Local Government Association said: "The use of management consultancy is commonplace in all sectors.
"Local authorities are multi-million pound businesses that offer hundreds of everyday services to communities.
"However, there is a limit to the specific professional expertise that is available to them within their authority.
"The term 'management consultants' covers a plethora of professional and technical expertise, advice and support that can help local councils to deal with a particular issue at a given time.
"It would be very difficult and expensive for councils to permanently employ all the detailed experience and expertise they might ever need. The short-term use of the right management consultants can be a cost-effective alternative. "
Rhondda Cynon Taf and Pembrokeshire councils refused to supply the information because they said it would exceed the cost limit of a Freedom of Information request.
Cardiff, Newport and Anglesey councils did not supply the information requested within the time limit.
The issue of the use of management consultants in the public and private sector is discussed on Tuesday's edition of the programme.
Wales@work, Radio Wales, on Tuesday at 1830 BST.