Officers at the Mid North Coast Correctional Centre intercepted a large quantity of drugs concealed in a legal brief last month.
The 500 strips of buprenorphine, along with phone paraphernalia, were discovered by staff with the assistance of drug detection K9 Obi on March 31.
The phone paraphernalia seized during the routine inspection of incoming mail included three SD cards and a phone charger.
"A large amount of contraband (was) hidden in an inmate legal brief," a Corrective Services Spokesperson confirmed.
"Congratulations to the vigilant Mid North Coast Correctional Centre staff," they added.
The items were detected after staff noticed sections of the brief appeared to be glued together.
Buprenorphine is a prescription drug used as a replacement in the treatment of heroin and methadone dependence.
It is used in the Opioid Agonist Treatment programme in NSW prisons to help inmates who have a dependency on heroin and other opioids to aid in managing their addiction.
It is an addictive drug that's administered via injection to help stop inmates trading it in other forms.
Corrective Services NSW and NSW Police are investigating the incident at the Mid North Coast Correctional Centre. No charges have been laid.
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