The New Zealand Drug Foundation said HNZ owed an apology, and possibly compensation, to evicted tenants.
Housing New Zealand defended its actions today.
In a strongly-worded statement, spokesman Charlie Mitchell said the organisation "took great offence" at comments by "so called experts" about its used of the meth testing guidelines.
He contradicted claims that hundreds had been evicted, saying just five people had been kicked out in the last year because their houses were contaminated by P.
Mitchell also said HNZ was acting on the only guidelines it had at the time, and that it had never received any advice that the guidelines were being misused.
That claim runs counter to comments by the Ministry of Health.
Director of protection, regulation and assurance Stewart Jessamine said health officials had met with HNZ on a number of occasions to underline the limits of the meth-testing guidelines.
"Underpinning those conversations has always been the Ministry's view that the Ministry's guidelines only cover clandestine laboratories and this has been routinely pointed out," he said.
Jessamine said he was aware HNZ was claiming it was not officially advised, but added that there was little to be gained from re-litigating the situation.
Asked for records or notes of the meetings with HNZ, he said none were available.
The Ministry of Health issued new guidelines this week which said the acceptable level of meth contamination in houses where P had been used was 1.5 to 2 micrograms per square centimetre.
That is three to four times higher than the level HNZ had been using.
Mitchell said HNZ would re-tenant 50 state houses over the next few weeks as a result of the new guidelines. The level of contamination in these houses fell between the old guidelines and the new ones.
New Zealand Drug Foundation head Ross Bell said HNZ had been warned many times by scientists and others that its testing methods were inaccurate and unfair.
There were no baseline tests of state houses, he said, which meant some people had been evicted for P use by previous tenants, visitors, or others.
"At no point in time was that contamination ever able to be attributed to the tenants," Bell said.
He hoped the new guidelines would "pour some cold water" on the "hysteria" which had developed around meth-contaminated houses.
"I think they've created a perception that most of New Zealand's rental properties are somehow contaminated with met and everyone should go out and spend thousands of dollars on cleaning up their house."
The Government has moved to tighten the rules for meth-testing businesses out of concerns that cowboy operators are misleading homeowners.
The minimum cost for cleaning up a meth-contaminated house is around $14,000.