The foul odour of methamphetamine was initially disguised by the strong smell of steam-cleaned carpets and a fresh lick of paint.

But a few weeks after moving into the weatherboard house she is renting in Melbourne's outer eastern suburbs, Jacki Whittaker detected a putrid stench in one of the bedrooms.

"It smells like a cat has pissed in this room," she told Background Briefing.

"If I've got all the windows open and it's a really sunny day and there's a breeze, not so much, but if you shut the doors, it's definitely there."

The real culprit, tests would later confirm, was not urine, but the chemical remains of an illegal drug-manufacturing operation.

The previous tenant had been cooking meth throughout the house, including in the bathroom and kitchen.

Jacki Whittaker sought a second opinion after Meth Screen first tested her house for contamination. ( ABC News: David Lewis )

Jacki's adult son, Leo, was the first to suspect drugs were involved. He thought it was strange that insects were avoiding the property.

"I noticed a lack of bugs and flies," he said.

"Anything that came in would either die or would leave very quickly."

At Leo's urging, Jacki contacted a company called Meth Screen to have the house tested for possible contamination.

A technician was sent to their home to take samples from the stained walls, floors, and countertops.

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In what resembled a scene from the hit American TV show Breaking Bad, the technician slipped into a hazardous materials suit before entering the home where the plain-clothed Whittakers were waiting.

The technician placed a 100-square-centimetre template on each surface before using a swab to collect residue from the space within it.

Though they said nothing at the time, Jacki and Leo observed the technician swiping larger areas outside of the template.

In Australia, a reading above 0.5 micrograms is considered unsafe.

To Jacki's astonishment, Meth Screen's analysis found parts of her property were 3,000 times that limit.

Although scientists are yet to establish any link between meth residue exposure and human harm, the company told the family the results were extremely serious.

"They said the house was definitely uninhabitable, that it was quite dangerous to our health, and that we should leave as soon as possible," Ms Whittaker said.

Further testing raised more questions

Despite the urgency of Meth Screen's advice, Ms Whittaker was not quite ready to accept the findings, let alone move out.

"When those results came in I guess the sirens went off in my head," she said.

"This can't be accurate. I really didn't believe them."

There were discrepancies between Meth Screen's test results and another company's findings. ( ABC News: David Lewis )

Ms Whittaker began doing some research and was eventually put in touch with Brad Prezant, an environmental scientist with more than 40 years' experience in public health.

Mr Prezant carried out a second test on the home. There were some major discrepancies between his findings and Meth Screen's.

"We sampled this area," Mr Prezant said, pointing to a photograph of a tile on the floor.

"We found 1 microgram, they found 77."

Another of Mr Prezant's samples, this time of the ceiling, turned up just 12 micrograms of meth compared to Meth Screen's 1,600.

"I have no explanation for that," Mr Prezant said.

These inconsistencies are not an anomaly.

Background Briefing has obtained confidential evidence from nine more properties examined by three different qualified environmental consultants. They were each hired to reassess homes initially tested by Meth Screen.

In all nine cases, Meth Screen's results showed contamination levels between 28 and 250 times higher than those subsequently captured by the environmental consultants.

Jacki Whittaker says the stress of living in a contaminated home keeps her awake at night. ( ABC News: David Lewis )

In perhaps the biggest discrepancy of all, one home was found by Meth Screen to have levels of residue consistent with an active drug-manufacturing lab.

But when a different company later tested the same house, it found the amount of meth was so low as to be undetectable.

Testing houses for meth became cottage industry in NZ

Across the Tasman, New Zealand has been grappling with how to remediate homes potentially contaminated by meth.

In 2013, the Department of Housing introduced a regime of regular testing at a cost of $120 million over five years.

More than 800 public housing tenants were evicted on suspicion of having damaged the properties through drug manufacturing.

The government even took some of them to court to recover costs and banned them from living in taxpayer-funded homes.

But everything changed upon the release of a landmark report last May into the health risks posed by exposure to meth residue.

A fresh coat of paint and steam-cleaned carpets briefly disguised the lingering smell of meth in the home. ( ABC News: David Lewis )

Professor Sir Peter Gluckman, the chief scientific adviser to the NZ Prime Minister, found no evidence that living in a house formerly used to manufacture drugs can make people sick.

"There's absolutely no evidence in the medical literature anywhere in the world of anybody being harmed by passive exposure to methamphetamine at any level," he concluded.

As far as meth-testing companies were concerned, the Gluckman report was bad for business.

It recommended a new safety threshold of 15 micrograms per 100 square centimetres in dwellings where meth was being used, rather than cooked.

That is 30 times higher than the Australian guidelines.

The number of homes in New Zealand needing remediation suddenly fell by well over two-thirds.

The Whittaker family was told it was unsafe to remain in their home. ( ABC News: David Lewis )

The industry responded by seeking to discredit the Gluckman report.

"We had attempts to suggest that we were withholding evidence or ignoring evidence," research analyst Dr Anne Bardsley said.

"It was a constant attack to keep themselves in a working position so they still had an argument to clean houses and test houses for meth."

Dr Bardsley came to believe many of the companies critical of the report had been operating using a business model relying on misleading customers.

She reviewed the work of one of those companies.

"They didn't follow any kind of template, they just tried to find as much methamphetamine as they possibly could in these readings and tried to really scare the tenants," she said.

That company was Meth Screen.

Meth Screen's website claims invisible meth residue found on surfaces and belongings harms human health. ( ABC News: David Lewis )

Dr Bardsley said the business is one of many that has since expanded to Australia, sensing an opportunity to make money in a country with a much lower threshold for meth contamination.

"We do know of very unscrupulous activities that went on in New Zealand through people who used to be in the industry who suggested that it wasn't all above board," she said.

"It's just wanting to shove that under a rug and see if they can build up an industry in Australia."

Meth-tester turned whistleblower speaks out

Dr Bardsley's concerns are well founded, according to a former Meth Screen employee in Melbourne.

After just one day of training in an industry without any guidelines to follow, Josh Long began testing homes for meth residue.

Mr Long assessed more than 40 properties for Meth Screen.

His colleagues nicknamed him Meth Hunter as the company could always rely on him to return a high contamination reading.

Former Meth Screen worker Josh Long says he felt uncomfortable with the company's testing methods. ( ABC News: David Lewis )

In hindsight, Mr Long acknowledges the methods he was encouraged to use do not sit comfortably with him.

"I don't feel so good about the way it's been handled," he told Background Briefing.

"We've almost deceived people by the way that we've come out with our readings.''

Mr Long said he would deliberately swab areas of homes he knew would contain high concentrations of meth, such as light switches.

The reading from those surfaces would then extrapolate to a general result for the home.

"I was using all of those figures to say, 'You've got stuff that's spread through your ceiling space through the entire building', and stuff like that," he said.

"It was to try to find as much meth as we can and then scare the crap out of them."

Mr Long eventually stopped getting work with Meth Screen and has now registered his own testing business.

Josh Long says he felt Meth Screen was deceiving its clients about the health risks posed by contamination. ( ABC News: David Lewis )

Meth Screen denies fudging the figures

In an interview with Background Briefing, Meth Screen's owner and director, Ryan Matthews, strenuously denied the allegations raised by customers, Dr Bardsley, and Mr Long.

"We would never tell anybody that a property was dangerous and to leave the property," he said.

"That is causing sensationalism and it's completely unrealistic."

Mr Matthews conceded the meth-testing industry is unregulated and full of cowboys, but insisted Meth Screen is a cut above.

"I'm a Kiwi from New Zealand," he said.

"I relocated to Australia in 2017 and carved ourselves out as being, I think, the leader."

Addressing claims Meth Screen testers regularly fail to stick to the 100-square-centimetre template when taking samples from homes, Mr Matthews said it was OK to deviate from a "black and white" approach.

"If you do it completely by the book, at times because of the uneven disbursement of methamphetamine in a room, there's potential for you to not collect the sample or collect enough residue to determine that it's contaminated," he said.

"I could provide it 100 per cent correct and then there's no meth there and everyone is going to be accusing us of not doing our job properly because we can't find it."

Mr Matthews also called into question the companies that re-examined Meth Screen's work, returning significantly lower results.

"Who are these people? What's their process? How are they trained?" he asked.

"Are they a competitor that's actually trying to discredit Meth Screen directly?"

Frightened family contemplates moving house

Ms Whittaker has begun packing her belongings into boxes in the kitchen of her contaminated rental home.

Her son Leo has already moved out, finding it too difficult to sleep in the bedroom with the strong chemical smell.

Jacki Whittaker says she will have nowhere to go if eventually forced out of her contaminated home. ( ABC News: David Lewis )

Ms Whittaker is herself struggling to rest.

"It keeps me up at night," she said.

"It makes me stressed and I eat too much junk food because of it."

If nothing else, Ms Whittaker hopes Australia will take note of what happened in New Zealand and refrain from introducing regular meth-testing of houses.

Her experience with Meth Screen has left her feeling suspicious of the industry's agenda.

"They're trying to promote their business and also change government policy in their favour," she said.

"They're relying on the fact it's a complex subject and if you're not educated, you're not going to know any different.

"If that's your business and every single house that's rented out or every house that's up for sale needs to be meth-tested, that's a gravy train for you."