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The proportion of southern prisoners failing drug tests has spiked despite the national number continuing to dwindle.
Statistics released to the Otago Daily Times under the Official Information Act have revealed the number of prisoners failing random drug tests in the nation's prisons.
In the 2015-16 financial year, 146 positive drug tests were recorded nationally out of 4233 random tests - a failure rate of 3.45%.
This was the lowest rate of failed random drug tests in prisons in the three years of data obtained by the ODT, down on 3.69% the year before and 3.83% in 2013-14.
However, Otago Corrections Facility and Invercargill Prison's failure rates rose respectively to 2.25% (from 0.53%) and 5.26% (from 2.9%).
Manawatu Prison had the worst failure rate in each of the three years of data with 14.84% of random tests returning positive results in 2015-16.
Department of Corrections chief custodial officer Neil Beales said drug taking in prison was an insidious trade, often led by gangs.
Corrections data showed drugs were confiscated from Otago Corrections Facility prisoners 23 times in 2015-16 - the highest number since 2010-11.
''Prisons are basically microcosms of society in many ways,'' Mr Beales said.
''If there's a particular spike in something outside prisons, whether it's methamphetamine, we will likely see a similar spike inside prisons.
''A lot of the prisoners have got a long history of using or abusing drugs or being involved in the drug trade, and that doesn't stop when they come inside prison.
''The rate of positive drug tests has fallen. But you don't take your eye off the target.''
Corrections were using a variety of methods to disrupt the flow of drugs into the prison system, including detector dogs, car searches, background checks and surveillance cameras.
''Very importantly, it's about having a very, very good intelligence network operation,'' he said.
''They are deployed across our sites. So we keep track of whose doing what on the outside, what are the trends.''
However, gangs continued to play a big part in organising drug smuggling behind bars.
Many times prisoners would smuggle drugs into prison internally. In 2011, OCF remand prisoner Jai Davis died as a result of an overdose of codeine and valium pills he attempted to smuggle internally into the jail.
''The gangs control a lot of the movement of contraband, whether that be drugs or other kinds of contraband,'' Mr Beales said.
''That can reach out into the community.
''There's pressure on these people to do these things.''
Many times visitors were pressured into performing as mules.
''When you speak to visitors, this isn't what they want to be doing,'' he said.
''[But] addicts will go to any lengths to feed their habits.''
Gangs and drug dealers would also use simpler means, such as throwing packages over prison walls under the cover of darkness.
If caught, those attempting to smuggle drugs into prison were often referred to police, he said.
''It's very much an underbelly of prison life that drugs are always going to be a threat to us,'' he said.
''They are a scourge in prison. They cause immeasurable damage to health, to people's families and to those smuggling them in.
''I have not yet come across a country that has eliminated it.''