Authorities say Tonga needs two more prisons and many more psychiatric beds to cope with offenders.
RNZ Pacific correspondent in Nuku'alofa, Kalafi Moala, said this was the view of the Commissioner of Prisons, Semisi Tapueluelu.
He said the issue of psychologically-disturbed prisoners had soared as the country battled a surge in drug offences.
A newly established psych unit at the Hu'atolitoli Prison already had 14 inmates. The unit was built for a maximum capacity of eight people.
Tapueluelu said there were many others who needed the services.
Kalafi Moala said there was a corresponding surge in the number of convictions being suspended by the courts.
"There have been so many court cases, particularly of people tried for drug use and their sentences get suspended and of course we asked the reason 'Why are these sentences suspended' and the Superintendent of the Prisons [Semisi Tapueluelu] gave the answer is because they don't have any room to take in new prisoners."
Over the years mental health officials in Tonga have drawn attention to the need for more facilities.
The local Mental Health and Disabilities Association said in the past that close to 10 percent of Tongans suffer from mental illness, yet the government had not always provided the necessary support.
Earlier Tonga's head psychiatrist, Mapa Puloka, said the sector was in dire need of attention with some psychiatric patients sleeping on hospital floors or not in official facilities at all.
He also pointed to a growing drug problem and a lack of help for addicts.