No one wants to buy a methamphetamine-contaminated home. Meth (aka P) use has become a scourge of New Zealand society.

More and more homes are testing positive for residues of the drug and/or toxic chemicals used to produce it.

The problem is that cleaning can cost tens of thousands of dollars. It's not just the cooking of meth that causes contamination.

Homes where meth has been used can also be contaminated and fail Ministry of Health guidelines. Buying a contaminated home, or even one that has been cleaned, can be a costly mistake, if it deters future buyers.

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So what can home buyers do to protect themselves?

Building inspector Matt Mason of Betta Inspect It recommends all clients tack on a basic meth test for $159 plus GST ($182.85).

Swabs from those tests, are sent to Hills Laboratory, the same lab used by Fair Go, which has criticised both government rules and private firms working in the field.

Mason says about 80 per cent of clients get the meth test done and of those 35 per cent get a positive indication, which then needs further testing to determine the location in the house of the problem and the exact levels.

If the levels exceed Ministry of Health guidelines the home must be decontaminated.

And if the council or other agencies are alerted, the contamination is recorded on the LIM, which all buyers should check.

Meth labs are more likely to be found in rental properties, but plenty of owner-occupied homes have become contaminated.

"Meth [use] covers a cross-section of the population from businessmen to the homeless," says Mason.

He cites the case of a grandson who had to admit to smoking meth in his 90-year-old grandmother's upmarket home after it failed a test. It's not an uncommon scenario.

Tens of thousands of properties have been contaminated over the past 20 years and buyers should always ask the agent directly if the house has been meth-tested by current or former owners or tenants and what the results were. Agents are duty bound to disclose this information if they know it.

As well as testing, checking the LIM and asking the agent, another useful technique in discovering a property's defects is to knock on neighbours' doors and ask questions. Sometimes they're all too happy to spill the beans.

To raise alarm bells the house doesn't even need to be a tinny house. Repeated partying could be a warning sign of repeated meth use.

A positive meth reading is a nightmare for a vendor. Decontamination firms charge wildly different figures for work that can involve stripping the home of walls and floor coverings, says Mason. And even once the home is "clean", meaning below Ministry of Health guidelines, buyers may be in very short supply or bargain hard on the price.

Valuer Steve McNamara of Property InDepth says if valuers find out there has been a positive reading, the valuation will be reduced by thousands of dollars.

An even bigger problem for vendors is that the spectre of meth scares off 80 per cent of potential buyers.

Just to make things worse for vendors, insurance policies won't pay for the clean-up of owner-occupied properties and only some rental property insurance policies do.

Some will still sell.

The Devonport Flagstaff newspaper reported that a three-bedroom villa that had been cleaned following a methamphetamine contamination still sold at a Barfoot & Thompson auction in early June for $1.56 million. How much more the vendors would have got without the meth reading is anyone's guess.

Currently Standards New Zealand is working on a New Zealand standard on the testing and remediation of properties.

This won't, however, change the Ministry of Health's guidelines on what's healthy and what's not, which have been queried by Fair Go.

That programme questioned whether owners were encouraged to spend tens of thousands of dollars replacing walls, carpets and throwing away belongings, when the readings were low.

The programme pointed out in June that most of the banknotes the programme had tested for meth carried higher levels than allowed in a house. It also questioned whether meth testing should distinguish between meth use and meth manufacture.

The latter was what carried the greatest health risk for anyone who lived in the property subsequently.

Property manager John May believes there is a lot of scaremongering contributed by a potentially rogue industry that is poised to benefit.

He questions whether LIMs should be tagged when the owners have done the right thing and cleaned up the contamination.

Finally, anyone buying a rental property needs to buy insurance policies that pay for decontamination should tenants contaminate the property.

Not all policies cover this and some of those that do, have limitations.

Unless you can read and understand every word in the small print, use an insurance broker to find the right policy for you.