The number of inmates returning positive results from random drug testing has fallen to an all-time low.
Corrections Minister Judith Collins has released figures showing just 6% of prisoners have tested positive in the financial year to date.
By contrast, more than 34% of prisoners tested positive when random drug testing was introduced in 1998.
Ms Collins is attributing the low figure to new laws targeting drugs, contraband and cellphone jamming and more vigilant prison guards.
Allowing some staff to read non-privileged mail to prisoners has also helped, as have Drug Treatment Units across the country, she says.