Northland is still the cannabis capital of New Zealand, according to a police report on a drug seek-and-destroy operation held earlier this year.

The Annual Cannabis Operation report shows that Operation Jess, which took place in Northland from mid-January to mid-March, destroyed 16,307 cannabis plants — more plants than found in any other New Zealand Police 2018 national drug eradication programme.

While there is a ledger of figures and values that can be applied to Operation Jess's findings, there are hidden costs associated with drug use, supply and related crime that can't be counted.

Those include the social, health and education costs of the impact of drug crime on innocent bystanders like children and other family members, Acting Detective Senior Sergeant Shane Pilmer said.

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''During Operation Jess there were five concerns reported that led to referrals to Oranga Tamariki (child welfare services),'' Pilmer said.

''You can't put a figure on the personal or long-term social costs.''

Operation Jess also resulted in several referrals to Te Ara Oranga, the fast-track system for methamphetamine addiction treatment which police and Northland District Health Board have pioneered.

Overall, the numbers of plants, other drug crime findings and charges were down on the 2017 drug eradication programme due to Operation Jess coinciding with the arrival of two ex-tropical cyclones.

Heavy rain grounded several days of planned aerial surveillance during that vital phase of the operation, Pilmer said.

However, the destruction of more than 16,000 plants was far ahead of the next highest regional find of 10,859 in the Eastern Police District which extends from East Cape to southern Hawkes Bay.

During Operation Jess Northland police seized 6.9 kg of dried cannabis, 24.95g of meth, nine firearms, $11,675 in cash and $193,000 of stolen property, including a truck, trailer and digger, and stolen cars.

Twenty seven warrants were issued prior to searches and 59 searches were carried out invoking the power of a warrant.

Several commercial-scale indoor cannabis-growing operations were uncovered, the biggest containing 293 plants at various stages of growth in a shed near Kaitaia.

While meth charges were laid, no laboratories were found.

''It was cannabis we were looking for in terms of the air operation but at the same time we made a few arrests targeting meth,'' Pilmer said.

''Normally we would stumble across one or two labs, but this time we didn't.

''When we came across meth, though, our police meth team followed up with families, referring them to various agencies they deal with.

''We took a two-pronged approach. One was detection, the other prevention.''

Pilmer said drugs were a huge part of policing in Northland because it was a major driver of crime, whether cannabis or methamphetamine.

''Meth in particular is seen as a scourge because of what it does to people and how quickly it becomes harmful, and the social issues and crimes associated with it.

''Meth is being driven by the gangs who control its manufacture and distribution.''