Flaws in prison health systems, including sick patients allegedly being left unattended, mean taxpayers have to foot a bigger bill when the prisoners are released.
These issues are revealed in a damning 157-page report by the nation's watchdog entitled Investigation of the Department of Corrections in relation to the Provision, Access and Availability of Prisoner Health Services.
Areas of concern highlighted by Chief Ombudsman Beverley Wakem and David McGee include:
* Unused medication wasting taxpayer money.
* A lack of resources to provide adequate dental services.
* Poor management of mentally unwell prisoners.
* Prisoners being denied prescribed medication.
* Paracetamol being "dished out like lollies".
Howard League spokesman Jared Gilbert said money should be spent on prisoners' health to avoid an inflated taxpayer bill when prisoners were released with aggravated health problems.
"We have a burgeoning prison population which needs to be dealt with and these problems will come back to haunt us if we aren't putting money in the right areas now," Gilbert said.
Inmates reported their frustrations around long waiting lists, patient neglect and being denied prescribed medication.
A Hawke's Bay Regional Prison inmate claimed he was declined access to a nurse after raising concerns about a decaying and discoloured tooth. A painful abcess developed but he was still denied treatment because he was "getting out soon".
Another inmate had a hernia operation and claimed his pain- relief medication was taken off him when he returned to prison from hospital.
Bodybuilder Justin Rys knows the prison healthcare system too well after being locked up twice for importing the illicit drug GBL in 2006 and last year.
The 116kg man has dilated cardiomyopathy, which causes his heart to fail about once every six weeks.
He also has megarexia, a mental condition which makes him think his body is small - despite being used as a model to simulate the giant gorilla for Peter Jackson's film King Kong.
Rys said he relied on a sleep apnoea machine which helped him breathe while sleeping.
But he claimed prison staff ignored his pleas for a replacement when the machine broke down.
"I couldn't sleep for two and a half weeks ... it was literally killing me," Rys said.
"It wasn't until [lawyers] threatened legal action and got in contact with the head of the prison that, magically, the next day I was able to go to hospital and get my machine."
Rys claimed another prisoner had testicular cancer and his blood-soaked bandages went unchanged for days, while another was denied pain relief after having cancer growths removed from his face.
"They are so low on numbers for nurses and doctors," Rys said. "They are trying to run a ship on a skeleton crew. I know we are in prison but we are still people."
The ombudsman's investigation found there was only one medical officer at a prison with 666 inmates who was contracted to work four hours a week.
Another officer worked three hours at a different prison with 112 inmates, while a third officer worked 20 hours a week at another prison with 280 inmates.
Department of Corrections acting national health manager Bronwyn Donaldson said prisoners' claims should not be taken as gospel.
"Prisoners are not always the most truthful of people in the country ... we weren't given the opportunity to verify the stories told," Donaldson said.
"If someone presented significant pain that wasn't taken care of by Panadol we would look to escalate that [level of care] as quickly as possible.
"We are really comfortable with the level of care we are currently providing."