More than 10,000 prisoners have faced charges for drug-related offences in the past four years.

The figure was revealed by Corrections Minister Matt Robson in response to a parliamentary question from Act MP Rodney Hide.

Random drug testing of prisoners suggest about a fifth of all prisoners continued to use drugs while inside.

Of the 10,271 inmates who have faced drug charges since 1998, almost 1000 were in Auckland's Paremoremo maximum security prison.

The charges related to 24 types of drugs, ranging from home brew to LSD.

The figures also said 204 cannabis plants, with an estimated street value of almost $1 million, were found growing at Rangipo Prison, near Turangi, in 1998, along with a cache of cannabis leaves and oil worth about $20,000.

Corrections Department officials deny the crop was grown by prisoners.

Mr Hide said the information showed prisons were "drug supermarkets".

"This speaks volumes about our drug policy in general," he said.

"These people were not sent to prison to take drugs and presumably some of them were serving time for drug offences.

"So if you can't keep them away from drugs with four walls and guards, what does that mean for the streets of Auckland?"

Mr Hide is calling for zero-tolerance of drugs within prison, although he does accept they make offenders more "malleable", which could assist prison guards.

A spokesman for the Public Prisons Service said most of the charges followed the introduction of random testing in 1998 and incidents had since reduced.

He said many prisoners brought their drug habits in with them and would use any number of means to maintain those habits.

Drugs had been found hidden in mail, personal belongings and had been delivered by visitors and even prison staff.

Those prisoners caught with drugs face penalties, including loss of privileges, cell confinement, and denial of early release.