Two men have been arrested and assets worth $5 million seized after two police raids, one of which discovered a "very large underground bunker" for cannabis growing, in the South Island.

The bust is the result of a covert investigation into the commercial cultivation and widespread supply of cannabis throughout the West Coast and Canterbury.

Farm and residential property, cash, bank accounts, vehicles and gold claims (permits to explore/mine) worth about $5m were seized by police.

A 72-year-old Ngahere man was arrested on September 9 and a search warrant executed at his address.

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He has been charged with possession for supply of cannabis and is due to appear in Christchurch District Court on September 30.

Yesterday, police from around the Tasman District also executed a search warrant at a large rural property in Kaniere, where it is expected they will remain for days.

There police discovered a large underground bunker containing a sophisticated cannabis-growing operation. A large number of firearms were also seized.

A 63-year-old man, charged with cultivating and selling cannabis, is due to appear in the Greymouth District Court today.

Inspector Jacqui Corner, West Coast Area Commander, said the operation goes some way to reducing harm in West Coast communities.

"Police rely on tips from the public to stem the flow of drugs, and information supplied from our community, which has been instrumental in police being able to prosecute these offenders and take their assets," she said.

"I would like to thank the West Coast community for their patience and information that they have supplied police over the past year about these addresses.

The scale and seriousness of the offending required careful investigation to put them out of business for good, Corner said.

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"Our investigation team has spent months working to dismantle this drug supply, and in doing so have prevented significant harm reaching our communities.

"Drug use is a driver of numerous other crimes so operations of this nature will continue to be a priority for Tasman District Police

"As well as holding people to account, police will ensure that those who are reliant on illicit drugs are referred to the relevant agencies to ensure they get the support they need.

"Let's work together to ensure we stop drugs causing more harm to our communities by continuing to be vigilant to this type of behaviour."