A crackdown on illegal mining has landed six gold prospectors in hot water.
All of them have been put out of business in Westland and Southland, and prosecutions are following in two of the six cases.
This results from a new policy from New Zealand Petroleum and Minerals, which brings to an end decades of turning a blind eye to people helping themselves to the property of other people or companies and of the state.
The Crown agency has appointed a former police officer to do its checking for it.
Its national manager for minerals, Ilana Miller, said illegal miners exploited the efforts of legitimate mining companies that had spent time and money on getting approvals to hunt for gold.
Those companies would find their work was being taken advantage of by opportunistic neighbours sneaking onto their claim area.
"The [illegal miners] were either operating on conservation land or in areas where people already had appropriate consents and permissions in place," she said.
"These individuals were going in there and taking advantage without the appropriate consents and permissions."
Adrian Field, a Christchurch mining engineer who did work on the West Coast, described how these fly-by-nighters operated.
"They go into land, which is on the face of it vacant," he said.
"They do not bother to check out the mineral status of the land, they put a digger in there and start treating it.
"It is not until someone goes past that might know the landowner and this sets off some warning bells."
Under long-standing law, gold is the property of the state, even if it is under private property.
In addition, the gold-seeking permits acquired by legitimate companies are worth a lot of money, so illegal gold mining amounts to theft either way.
Mr Field said it could be profitable theft for the perpetrators.
"If they happen to get onto the right sort of deposit you are probably talking about hundreds of thousands of ounces, depending on how long they get away with it before someone blows the whistle on them."
With gold worth over $1800 an ounce, that can be a lot of money.
But Chris Baker of the mining lobby group Straterra said he thought illegal mining was a hit-and-miss affair, done by small timers who would be unlikely to make big money out of what they did.
Nevertheless, he had a simple message for them: they were breaking the law and there was no excuse for that.
Until New Zealand Petroleum and Minerals got tough on this, very little was done about it.
Ms Miller was adamant her organisation was doing the right thing with its crackdown, but she conceded ending illegal mining for good would not be easy.
"This is just our first year in looking into this, and a lot of places where this is happening is every remote.
"So a lot of places where it is happening are hard to detect, so this is just the start."
Illegal mining is punishable by up to two years in jail or a $400,000 fine.
The mining operations in question all involved alluvial gold.