Members of the public are being asked to report suspicious behaviour to the police as the cannabis growing season could see some bumper crops this year, provided growers can keep their plants watered.
The season runs from November to March.
Frequent trips into the bush or rural areas could be a sign that visitors are not just trying to connect with nature.
Detective Patrick Derbyshire, of the police Organised Crime Unit in Kerikeri, said cannabis plants would be flourishing in the current hot, sunny weather - but they would need regular watering.
"With no rain the growers will have to go in and physically water them. We're asking people to report any suspicious behaviour, like cars parked in rural areas, people frequenting the same out-of-the-way places, or going in and out of the bush, especially if they have no other reason to be there. They won't hesitate to use your property."
Mr Derbyshire asked anyone who saw suspicious activity to take note of the location, the car's registration, and if possible a description of the car and its occupants, then call the nearest police station.
Call Whangarei police station on 430 4500, Kerikeri 407 9211, Kawakawa 404 3150, Kaitaia 408 6500, Kaikohe 405 2960 or Dargaville 439 3400. Calls can also be made anonymously to Crimestoppers on 0800 555 111.