A state house tenant was evicted from her home and stung for $20,000 over a now-discredited meth test and because she had a relative who "may" have been involved with drugs.

Photo: RNZ / Claire Eastham-Farrelly

The woman was among dozens of state housing tenants the Tenancy Tribunal ruled were responsible for methamphetamine contamination, using evidence that legal and toxicology experts say was insufficient.

RNZ examined the last 100 Tenancy Tribunal decisions where Housing New Zealand (HNZ) accused tenants of contaminating properties with methamphetamine.

Of those decisions - issued between August 2016 and May this year - 86 accepted HNZ's evidence, ordering the tenants' eviction and often costs in the tens of thousands of dollars.

In the few decisions dismissing HNZ's claims, the tribunal criticised the level of evidence the agency provided, with one calling it "sparse and vague".

But in many other cases, tribunal adjudicators accepted a similar level of evidence as enough to find tenants responsible, on the balance of probabilities.

The tribunal has defended its approach, saying it had "always relied on the most relevant and current guidelines available for its assessment of methamphetamine contamination claims".

A report by the Prime Minister's chief science adviser Sir Peter Gluckman last week found there was no health risk from living in a house where meth had previously been smoked.

Since 2015, HNZ has successfully sought hundreds of Tenancy Tribunal rulings stating houses with traces of meth were "uninhabitable" - and that tenants therefore had to leave.

The tribunal has a strict window of 10 working days to lodge an appeal, meaning none of the cases can now be legally challenged.

In many of the cases the only evidence presented were test results - which toxicologists had already questioned - indicating there was methamphetamine residue on surfaces in the house.

Often, there was no "baseline" test to show whether the meth was already present at the start of the tenancy.

The test results were sometimes combined with circumstantial evidence, such as tenants' demeanour and reports of "multiple vehicle traffic".

'I'm lost - I don't know what to do'

Photo: RNZ / Claire Eastham-Farrelly

One former tenant, an Avondale woman with four children, was ordered to pay $20,500 after the state house they had lived in for 12 years tested positive for meth.

The house had not been tested to make sure it was meth-free before the family moved in, but the woman had told the tribunal that a relative who had stayed at the house might have been involved in drugs, though she had never seen him use them.

That was enough for the tribunal to find it more likely than not that she was responsible for the contamination - and the cost.

She told RNZ she felt "sad, angry and depressed" about losing her house - which was the only home her youngest daughter, now 13, had ever known.

It took five months for the family to find a private rental big enough for them all to live in.

In between, the family had to separate.

"I was sleeping in the car, just moving from here to there; my older girl had to move in with her aunty and my youngest moved in with my mum.

"For the very first time, we were separated… It was hard - really, really hard."

The money she now owed was "too big to think about", she said.

In a written statement last week, HNZ chief executive Andrew McKenzie said the agency would not be seeking further payments from tenants.

However, the woman said HNZ had not been in touch about the money she owed, or about the report into methamphetamine testing.

"At the moment I'm lost - I don't know what to do."

Other decisions against tenants included:

A woman who "had a known history of drug use by the NZ Police" (ordered to pay $21,000)

A woman who was found at fault because her partner had been found responsible for the contamination of another house ($21,000)

A man who was blamed because of anti-social behaviour and "evidence that gang members were often at the property" ($17,000)

In all those cases, adjudicators said that was enough, on the balance of probabilities, to hold the tenant responsible.

Otago University law lecturer Marcelo Rodriguez Ferrere, who specialises in administrative law, said from the cases he had seen, the evidence HNZ had provided to the tribunal did not even meet that test.

"You usually need a sufficient amount of evidence that's clear, substantiated and causative of the facts you're trying to prove before it's going to be reliable," he said.

"And it seems in some [of these cases] just to be too circumstantial to be relied upon."

In one of the few cases dismissing HNZ's claims, a 2016 decision found that although test results showed contamination, HNZ "provided no evidence to show that the property was not contaminated at the start of the tenancy".

Syringes and a pipe found at the house had not been tested and there was "no proof" they had been used to consume methamphetamine.

Another finding against HNZ called the evidence "sparse and vague" and said HNZ was making inferences "based on conjecture and generic observations".

Positive test results should have been scrutinised - toxicologist

Victoria University property law lecturer Mark Bennett said there was a "variability of scrutiny" in the decisions.

In cases where there was a negative test before the tenancy and a positive test afterwards, the question seemed to be, 'How else would the meth be there?', he said.

"That seems to be the logic of tribunal decisions with little discussion, and … it is arguable that is all they need to do," he said.

Photo: Massey University

However, Massey University toxicologist Nick Kim said there were many ways the test results could be incorrect or invalidated.

"[There's] a whole range of things that an expert would ask … that were probably not asked in a range of the tenancy cases."

That included showing there was no cross-contamination and that the testing company could produce a 'field blank' - a test taken that showed a zero result - to prove they could take a sample without getting a false positive.

"It'd be pretty easy to dismiss many of them," Dr Kim said.

However, without the capacity to question the test results, the tribunal was probably "hamstrung" by the Health Ministry guidelines, he said.

Some adjudicators were aware of concerns over the guidelines.

One conceded they "may not be an ideal guide … but they are all that is available in New Zealand at the present time".

Principal tenancy adjudicator Melissa Poole said the tribunal had amended its approach to methamphetamine cases as the law and scientific evidence had evolved.

"All adjudicators receive ongoing training and education about methamphetamine contamination and Professor Gluckman's report ... will form a part of future training."

Neither she nor Associate Justice Minister Aupito William Sio responded directly to questions about whether the tribunal would review its handling of methamphetamine-related cases.