Farm owners were urged to be vigilant and do reference checks before letting anyone stay in their properties.

More and more houses for dairy farm workers are being used to manufacture meth.

A meth-testing company, Federated Farmers and the police all say their isolation isn't the only thing that makes them perfect for cooking up the drug. There's also the easy access to chemicals on a dairy farm, plus the crazy hours that farm hands can put in.

"It's a bit of a perfect storm for them. They are living in secluded areas and a number of precursor chemicals are available in the farm sheds," meth-testing company SDF Property Inspections owner Simon Fleming said.

Dairy workers are also working long, hard hours, he said.

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"Some use drugs, like meth, to get through the day, as they manufacture it because they have access."

Methamphetamine is made by mixing various chemicals. Pills for cold remedies are often used as the basis for the production of the drug.

The meth "cook" extracts ingredients from those pills and to increase its strength, combine the substance with chemicals such as battery acid, drain cleaner, lantern fuel and antifreeze, many of which are commonly found in dairy sheds.

At a recent Federated Farmers meeting, the issue was raised in conjunction with a rise in positive farm worker drug-tests, Federated Farmers Waikato president Chris Lewis said.

"It's starting to happen a lot," Lewis said.

"We have had a few members write to us and speak to us about it, raising concerns."

When methamphetamine is cooked in a house, the toxic fumes from the chemicals infiltrate everything, including the walls, curtains, carpets and furniture.

The resulting pollution can cause acute health effects, including asthma, skin rashes, eye irritations, headaches, nausea.

"Look," Lewis said, "90 per cent of farm workers will be the hardest working, most honest, decent people. There is just a handful of people who aren't."

He knew of a Waikato dairy farmer who drug-tested his staff recently and 75 per cent returned a positive result.

If meth houses kept popping up on the dairy farms, it could have long-term implications for the employees, Lewis said.

Dairy farmers would start reconsidering offering the houses on their farms, he said.

"It's become a lot of risk and very little comeback."

He said more farmers are asking for information.

"They want to know what is the standard required, what they need to do and what is the standard of testing."

As a result, Federated Farmers is looking at putting together a nationwide roadshow to educate farmers around safety.

Coromandel-Hauraki Detective Sergeant Martyn Hughes agreed the ingredients are there for dairy farm houses to be used to cook meth.

"I'm quite certain that's the case."

He urged farm owners to be vigilant.

"Do reference checks before you let anyone stay at your properties. If you see people coming and going at all times of the day and night, go and see what is happening," Hughes said.

"Don't ignore the signs. It is your property and you are allowed to check on it if you are suspicious."

Keeping up regular checks on the property within contractual obligations was a good idea also, Hughes said.

You'll never stop people who want to do drugs, though you might stop them from manufacturing meth.

"If they can't do that, those types of people will always find different things to do."