Home buyers and landlords have been warned to watch out for a growing number of properties contaminated by being used as P labs.

About 1300 labs have been found in the past 10 years, and decontamination specialists believe the total number could be 10 times higher.

Cooking methamphetamine produces toxic chemicals that can poison the air in a house for years.

Homes often have to be stripped of carpets, wallpaper and floor coverings to make them safe again.

Several Weekend Herald readers described the damage and the health and financial costs suffered from unwittingly buying a former meth lab.

One person affected bought a house in Auckland in February and moved in the next month.

"Over a period of two months I developed worsening sleeplessness to the extent that at the end I was getting only about three to four hours' sleep a night," said the man, who did not want to be named.

"My sinuses were also really bad.

"At some point near the beginning of April I mentioned to my colleague at work that I was having trouble shutting down at night. I felt wide awake."

His ears, nose and throat grew unbearably sore and an unnatural chemical smell took over the house, so he contacted a drug detection firm to test the house for P residue.

The test was positive and the full cost of decontamination was $3000 - comparatively low for cleaning up a former P lab.

The home-owner said he was almost relieved but still had to pay $1100 for two initial tests and $1000 for a certificate of habitation which proves the house is safe to live in. He also felt frustrated because he thought he had checked the house thoroughly.

"When I first bought the house I did the three typical things as part of purchasing a house - building inspection, valuation, LIM report. I guess now house buyers also need to test a house for P residue too."

Another reader said he lost more than $15,000, including lost rent, when police found his tenants building a P lab in the back shed. The next tenants were P users, who trashed the house, causing $40,000 of damage.

"Friends caught our tenants stealing from a local building site and nabbed them, only to be confronted with a knife. Fortunately our friend could defend himself and got the bugger."

Nick McLeay, a former head of the police P lab response team in Auckland and now director of the NZ Drug Detection Agency, said police were finding about 90 labs a year in the greater Auckland area alone.

Based on unofficial estimates that police discovered only about 10 per cent of the labs - which he thought could be right - there were probably thousands of contaminated properties.

Auckland Property Investors Association president Sue Tierney said none of her 800-plus members, who owned at least 5000 properties, had reported a problem with a P lab.

But the association was well aware of the risk and suggested precautions for landlords: Choose tenants carefully and get references.

Ask for rent in advance and get at least two weeks' bond.

Get the right insurance cover.

Inspect the house at least four times a year and don't accept requests not to go into certain rooms.

33 per cent of P-lab houses had children living in them.
3 out of 4 children were present during the cooking process.
40 per cent were found in drug busts.
22 per cent were discovered by the public.
33 per cent had weapons inside, including firearms and explosives.
Source: National Drug Intelligence Bureau