In Anglesey, not one of the 11 practices in the county is able to take on any new patients and in Neath Port Talbot all 17 NHS practices are full to capacity.
Meanwhile, in rural Ceredigion, only one of the 33 practices has any spaces.
Waiting lists in some areas are so backed up that it takes several months to register with a NHS practice and then as long as another 18 months to get an appointment for a check-up.
Carol Lamyman-Jones, director of the Board of Community Health Councils said the problems persisted due to a continuing shortage of NHS dentists.
She said: ‘Wales has, over the last few years, experienced a mass exodus of NHS dentists converting to private practice and the losers are quite obviously, the public. We witnessed a dental crisis back in 2006 where numerous people were being asked to travel vast distances to receive any NHS treatment. Anglesey residents, for example, were forced to travel all the way to Liverpool. That is just not acceptable.
‘Things have improved slightly, but a great deal more needs to be done to ensure the availability of an NHS dentist in all communities across Wales.’
Around 1.4m people across Wales do not receive any regular dental treatment and fewer than 150 practices are taking on new NHS patients.
Other areas such as Newport also have significant problems with only four of the 21 practices able to take new patients, and in Torfaen, only two of 14 practices are taking people on. In Pembrokeshire patients are all entered onto a database to be assigned a place with a dentist, with an average wait of 12 months to book a routine appointment or check-up.
But despite the shortages revealed by the Western Mail, a Welsh Assembly Government spokesman said the situation was improving.
He said: ‘Dentists are independent contractors. Thanks to extra investment by the Welsh Assembly Government, especially in areas where access had been historically lower than the rest of Wales, more people can now see an NHS dentist.