A drug detector dog at Manawatū prison sniffed out more than 11 grams of cannabis oil that were hidden in a pair of socks mailed to a prisoner.
The prison's acting director Graham Dack said the socks and drugs were handed over to police this week, and inquiries were underway to find out who sent them.
Drugs and other contraband created a dangerous environment for staff and prisoners, Mr Dack said.
"Being affected by drugs prevents prisoners from fully engaging in the education, employment and rehabilitation programmes that can help them live a life free from crime when they are released.
"It is disappointing when we see people try to push boundaries in order to get drugs or other contraband in to prisoners.
"We want people who try to introduce items into prisons to know that we will find them and you will face consequences."
There are 25 detector dog teams operating across the country and it was detector dog Zena who made this particular find.