Fewer prisoners are testing positive to drugs, according to figures released by Corrections Minister Judith Collins.
More than 4500 general random drug tests were carried out on prisoners last year and 3.1 percent returned positive results.
That compares to 10 per cent in 2009/10 and 22 percent in 2000/01.
"As well as being illegal, drug use undermines the safety, security and good order of our prisons," Ms Collins said.
"Corrections place considerable emphasis on reducing the supply and demand for drugs in prison."