A reduction in flying hours combined with wet weather reduced the number of cannabis plants police could potentially destroy during their annual operation in Northland.

But the destruction of 21,531 cannabis plants was still a significant amount of illegal drugs to take out of circulation, says the officer in charge of Northland's Organised Crime Squad Detective Sergeant Stephen Chamberlain, who headed the drug blitz targeting commercial cannabis growers.

"There are still large commercial crops being grown and we know from experience the bulk of cannabis grown in Northland is generally tied into organised crime like gangs," Chamberlain said.

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"Any amount