The number of cannabis plants seized in New Zealand each year is only a "small proportion" of those grown, police say.
A rise in plant seizures since 2000 reflected an increase in illicit growers and targeted action, they said.
More than 140,000 cannabis plants were seized across New Zealand in 2008-09 and 1157 people arrested.
In Canterbury, 66 people were arrested in 2009-10 and 1845 plants seized.
Nationally in 2009-10, the number of plants seized dropped to 118,000, and 912 people were arrested.
Detective Inspector Paul Berry, of the national crime investigation group, said the seized plants were only a fraction of those cultivated.
"It would probably be fair to say we get a small proportion of the potential cannabis harvest out there, both indoor and outdoor," he said.
"There's no way we can fly all the areas to get all the cannabis. We are really looking at areas that are known to be hot spots.
"There will always be users, let's be honest, and it will always be available because people move from outdoor to indoor."
Police said they could not estimate the annual cost of operations to seize cannabis or the number of officer-hours involved.
However, Berry said it was "worth the cost" to target commercial growers.
"We don't go out there hunting down people who smoke a cannabis cigarette in their lounge. We need to concentrate on organised-crime gangs and where the money is being made."
Police estimate each kilogram of cannabis grown creates a social cost of $11,790 and Berry said commercial growing operations were often linked to other crime.
"If you do your research about crime, drug use always features as a driver," he said.
- © Fairfax NZ News