Half of the New Zealand state houses tested for methamphetamine in the past two years came up positive for the drug and it might be just the tip of the iceberg, experts say.

Figures released to the New Zealand Herald under the Official Information Act show there were 101 state houses in 2014 contaminated with methamphetamine, also known as P, out of 196 that were tested.

In 2013, just 8 state houses tested positive for the drug out of 19 tested nationwide.The whopping hike in testing numbers meant taxpayers forked out $1.3 million cleaning up the contaminated homes last year - almost $1 million more than what was spent the year before.

Housing New Zealand (HNZ) spokesperson Bryony Hilless said the massive hike in numbers tested was simply a result of improved detection processes.

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The organisation had put a lot of effort into better information sharing with police and HNZ staff were becoming more experienced in identifying P use and contamination, she said.

"We believe there is also increased awareness about the presence of methamphetamine in the community resulting in possible contamination being brought to our attention," she said.

It costs between $600 and $1800 to test each house and up to $30,000 to clean them, HNZ said.

Meth Minder - a methamphetamine detection company - director Miles Stratford said there was likely to be a lot more then 101 state houses affected by methamphetamine.

HNZ owns about 68,000 homes throughout the country and up to 20 per cent of those could be affected, he said.

Met Minder tested 1200 rental properties, most of which were privately owned, in 2014 and 40 per cent came up positive, Mr Stratford said.

"That will be the same sort of thing existing in HNZ probably. Does it mean 40 per cent of HNZ property has meth in it? Probably not, but it could be as high as 10 or 20 per cent and that's still an awful amount of properties.

"We've already got a massive block of contaminated property which sits undiscovered in the housing market."

Mr Stratford said the methamphetamine contamination across state and rental houses was a real danger for new home buyers.

HNZ should be doing more comprehensive testing of all of its properties, he said.

"The view of the government is that they don't need to worry about it and maybe that's because only 196 house actually got tested."

Auckland Property Investors Association president Andrew Bruce said there was a significant P problem across both private and state rentals.

It was important to understand the range in the scale of P contamination in rentals, he said.

"There's a significant P problem out there and a lot of these people will use rental accommodation. It's very much a risk item whether it be for HNZ or for landlords.

"I suppose what we really have to understand is what is considered a reasonable level. There's going to be a massive difference between a house being used as a lab or someone having a smoke."

The HNZ testing was part of a nationwide sweep of 196 addresses and houses that tested positive came up with readings of 0.5 micrograms per 100cm2 or more.

Readings could be caused by anything from use of the drug to a fully fledged P lab set-up.

Auckland had the highest number of contaminated houses followed by Tauranga City with nine.

HNZ said the agency would never knowingly place a methamphetamine affected property on the market.

Despite this, homelegal.co.nz advised buyers to be aware of the issue and ask questions when looking to purchase an ex-state home.

Anyone with concerns about methamphetamine contamination at a Housing New Zealand property should phone 0800 801 601