Rumours of an underground bunker on the West Coast where $5 million in assets were seized have been circulating for 10 years, residents say. 

The large concrete underground bunker hiding a "sophisticated cannabis-growing operation" on the West Coast is at the centre of a multimillion-dollar bust.

A neighbour of the property said he reported his concerns to police about the property and the man's behaviour soon after moving to the area in 2012. 

Police said they had received information from the public for years about the drug ring, but it took until a few months ago to launch the operation that led to two arrests. 

A 63-year-old Kaniere man was arrested after a police raid on a large rural Kaniere property on Tuesday. He was charged with cultivating cannabis as well as selling it, and appeared in the Greymouth District Court on Wednesday. He was granted interim name suppression and released on bail. 

He declined to comment when contacted by Stuff on Wednesday. 

A 72-year-old Ngahere man was also arrested after a search warrant was executed at his address on September 9. He has been charged with possession of cannabis for supply and will reappear in the Christchurch District Court on September 30. 

Locals said they had heard rumours about a bunker on the property going back 10 years. 

The man has owned the large property since 1994. 

Westland mayor Bruce Smith said he was "staggered" when he heard about the bunker. 

"My understanding is the thing is huge. It's 24 metres long by 12m wide and was buried 7m underground. We're talking a substantial size," he said. 

He had spoken to people who had heard rumours about its existence for years.

A Hokitika man who declined to be named said he had heard rumours about a bunker 10 years ago. Another said he had spoken to a "thug" who claimed he had helped the man pour concrete into the bunker. 

A neighbour of the property, who Stuff cannot name for legal reasons, said he had been complaining to the police about the 63-year-old since 2012. 

"The police knew what was going on for years and did nothing," he said. 

Senior Sergeant Mark Rowbottom said police spent several months investigating the large-scale commercial cultivation and supply of cannabis throughout the West Coast and Canterbury.

"The information gathering stage has been longer than months, I would say years," he said.

"Information has been coming through from the public constantly. It took time to gather all that together. It took a lot of things coming together to take some action."

The arrests made this week led to $5m of assets being seized including farmland, residential property, cash, vehicles, farm machinery, gold claims and a large number of firearms.

"We believe we have made a significant dent in the supply of cannabis to the West Coast and Canterbury areas. 

He said police finished the scene examination at Kaniere on Wednesday afternoon. He declined to say how big the bunker was or how much cannabis was found. 

He did not rule out more arrests or charges being laid. 

West Coast Area Commander Inspector Jacqui Corner said tips from the public were instrumental in police being able to make the arrests and take the assets. 

The operation goes some way to reducing harm in our West Coast communities.  

“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," Corner said.

“With the scale and seriousness of offending police needed to carefully consider and thoroughly investigate the offending of these groups as well as their assets and put them out of business for good."