The Corrections Department says the number of prisoners testing positive for drugs in random checks has fallen to a record low.
However, it says the rise in methamphetamine use in prisons continues to be a major problem.
When drug testing was introduced in 1998, there were positive results from 34% of randomly tested prisoners.
The current national average is 13%, which the Corrections Department says is an all-time low.
The department's Karen Urwin says the drop is mostly down to increased security, including better surveillance technology and double the number of dogs working at prisons.
But she says smaller and more powerful drugs, such as methamphetamine, or P, continue to be an issue.
Corrections Association president Beven Hanlon agrees, saying P is harder to detect than cannabis, and that partly explains why the number of positive drug tests is lower.
He says most drugs get into prison through visitors, some of whom should only have non-contact visits.