ANNUAL OPERATION: Members of a police and Royal New Zealand Air Force team carry cannabis recovered during an aerial operation in the Wairau Valley in Marlborough.

It's about 11am when the air force Iroquois helicopter hoves into view over the Wairau Valley, east of St Arnaud.

It touches down in a paddock on an undisclosed farm and police officers jump out carrying bundles of bright green cannabis, freshly hacked with machetes.

There are more than 100 healthy young plants, which the officers load into a trailer. They will later be burned on a bonfire.

"You can see by what's in that trailer," says Detective Sergeant Chris Roberts, officer in charge of the Tasman Organised Crime Unit.

"Each one of those plants growing to maturity is a large amount of cannabis that we've just stopped going out into the street."

The annual police cannabis recovery operation concludes today and Roberts said the number of plants located across the Tasman district was up on recent years.

He expected about 4000 plants will have been recovered by today, about 500 more than last season.

It's not known whether that's because more cannabis is being grown, or police are getting better at finding it.

The plants in the back of the trailer were recovered on Friday morning at a location that was identified using information gathered over several months.

The first part of the operation is intelligence gathering, Roberts said.

Police collect information from informants, anonymous tip-offs, local people, the Crimestoppers phone line and other sources.

"Obviously we analyse what comes in, place the weight on it and build it up. Some of it needs more work to justify further investigation."

The second part of the operation is surveillance from a fixed-wing plane over areas that have been identified as potential targets.

During the recovery phase, ground teams and a specialist helicopter unit hit each target and remove the cannabis. Search warrants are needed to enter private land.

Roberts said outdoor growers tended to have "reasonably good bush skills" and some were hunters. "People from all walks of life get involved. Usually younger, fitter people are growing outdoors a little bit back and beyond the main road."

They often planted near a water source in an area with some bush cover or on private land with or without the owner's permission.

Roberts said outdoor growers were dealing cannabis for financial gain, rather than personal use.

"You don't grow 100 plants to smoke yourself. When you find growers that are growing to that extent there's no way that you could ever justify that it was for personal use only."

Sooner or later, cannabis growers get caught, Roberts said.

"Your number will be up one day."

Roberts said he was aware that people have mixed views about the criminality of cannabis use, but he believed it caused social harm.

"You ask any cop about cannabis, we all deal with it. It's just like alcohol, it's another problem that we've got in our community.

"We're not looking at the people that might sit at home and have a couple of joints because they often probably wouldn't come to police notice, but we're out there targeting the people that are commercially growing cannabis to put out on the street, whether it be for themselves, through gangs or whatever.

"We've got to try and eradicate it . . . We're certainly making a big dent."