The resident drug dog at Northland Region Corrections Facility deserves some doggy treats after finding a carefully disguised parcel of drugs in incoming mail.

Roxy and her handler found a tightly wrapped cannabis leaf concealed in the spine of a book in one package.

Acting prison manager Chris Gilser says Roxy is an extraordinary dog and a great help in their fight against drugs.

He says people are trying to get more creative with sneaking in drugs. "We've known mothers to bring children in with drugs in their nappy."

Mr Gilser also has a warning for those wanting to try and beat the system. "Eventually you're going to get caught and it's only the family of the prisoner that will suffer."

Mr Gilser says the police are now investigating prosecuting the sender of the most recent drug find.