The scientific evidence used by Housing New Zealand to evict people for methamphetamine contamination is being questioned by an expert in the field.

Photo: Contaminated Site Solutions

Some people have been charged large sums of money to cover the cost of decontaminating properties, based on that evidence.

Housing New Zealand has ramped up its testing regime, carrying out nearly 593 tests in the last six months of last year - with 279 homes testing positive for methamphetamine.

RNZ has obtained documents that detail the tests used in three of the most high profile evictions of the last few years.

Nick Kim is a senior lecturer in environmental chemistry at Massey University and he examined the test results from the three houses.

He said one of the houses was likely being used for methamphetamine production, but he had serious concerns about the other two, and the way the tests were carried out.

Dr Kim said test results from a state house in Whangarei suggest people there had smoked methamphetamine.

But he said the recommendation by the company that carried out the tests, Dowdell and Associates, that the house not be tenanted and decontaminated, was overkill.

"You need a bit more of an argument to say a property shouldn't be tenanted.

"When I look at the New Zealand guidelines they don't refer to de-tenanting houses - they refer to reoccupying methlabs."

Arguably the most high profile case is from a state house in Ashburton.

The tenant there was evicted and the Tenancy Tribunal ordered her to pay $34,000 for the cost of decontamination.

But Dr Kim is not convinced anyone at that house had even smoked meth there.

"From an analytical chemistry point of view and also knowing the traces of methamphetamine than can exist just in the community on bank notes, I know that it's easy accidentally to contaminate samples when they're being collected," Dr Kim said.

"With these results I could view them two ways, I could say two of the 11 sample results show presence of methamphetamine at the property, or I could say that two of the 11 represent sample contamination that was introduced - I certainly can't prove it's not contamination."

An investigation by TVNZ's Fair Go this year found meth contamination is so common it can be found on bank notes at levels tenants are being evicted for.

Dr Kim said typically at outdoor sites where contamination is being assessed against guidelines, the results would be averaged out but this did not happened at the Ashburton house - if it had it would be well below the guideline level.

The Tenancy Tribunal refused to comment when approached by RNZ saying it could prejudice future decisions.

Dr Kim said Housing New Zealand was going too far in kicking out tenants after detecting forensic level drug traces and the situation was becoming bizarre.

However, Housing New Zealand's chemical contamination manager Charlie Mitchell said there was zero tolerance for illegal activity.

"If we can determine that the tenant is responsible for the contamination then we're going to terminate that tenancy."

Housing New Zealand acknowledged the meth guidelines it applied to its houses was never intended to assess the smoking of the drug - and admitted it was "not entirely suitable" .

It is participating on a committee that will develop new national meth contamination standards.

Nevertheless, Mr Mitchell said the agency would continue to evict tenants in the meantime.

"But as I say when people are using meth within our properties, and when we're going through the appropriate steps to carry out testing ... it's appropriate for us to act on that."

Green Party MP Marama Davidson said some of the country's most vulnerable people were being left homeless on the back of dubious testing.

She wants a moratorium on evictions to be considered until proper guidelines are in place.

"Testing for P (meth) is yet another way for the government to rid itself of its responsibility to provide state housing," Mrs Davidson said.

Dowdell and Associates, which tested the Whangarei house, also tested the Ashburton property where the tenants were left with a $34,000 bill.

The scientist who carried out the testing refused to talk to RNZ, saying that was not allowed to under their contract.