Compliance with healthy homes standards is still lagging more than two years since the law took effect.
Advocates say it is leaving renters powerless as they get stuck in draughty, mouldy homes, amid rental shortages.
Since July 2021 all boarding houses and private landlords needed to ensure their rental properties met healthy homes standards within 90 days of any new or renewed tenancies.
From July 2024, all rentals must comply.
The standards are for heating, insulation, ventilation, moisture and drainage, and draught-stopping in a home.
Betta Property Compliance carries out healthy home inspections.
Chief executive Matt Mason said while things like insulation were passing, there were reoccurring areas of concern.
"So things like draught-stopping, we only had a 46 percent pass rate on that across the country on average, which is one in every two properties only passed that."
Mason said about two-thirds of properties passed the drainage and guttering check.
After an initial rush of property management firms getting up to date, he said it was now private landlords scrambling as new tenancies began.
But that is causing some problems.
"We're getting a lot of feedback from the market that they are struggling to comply in time due to the shortages and supply issues with getting things like heat pumps or insulation or moisture barriers or draft stops."
Home testing group All Clear cofounder Adam Gordon said he was seeing the same issues.
Its data shows 57 percent of homes it assessed were compliant with all five healthy home requirements.
Gordon said landlords urgently needed to get on top of things and not rely on their tenants staying put.
"So we've seen a lot of owners thinking that 2024 is the end date [and] the longer that tendency doesn't end between now and 2024 they've got all that time to remain to reach compliance, the reality is, no one really knows when a tenancy will end."
He said there needed to be more enforcement on private rentals.
"Relatively poor indication in the market around, you know directing private landlords and there's a lot of people that have simply put their head in the sand."
Renters United spokesperson Annie Bykova said with rental shortages in many parts of the country, it had become a lottery, which landlords decided to comply with the standards or not.
"It's pretty much in the hands of the tenant to go to the tenancy tribunal and start this big legal fuss about it because there isn't somebody that they could just sort of report it to that would investigate it without their full involvement."
That puts a lot on the line for people who need somewhere to live.
"We've even heard sort of anecdotal stories of landlords and property management companies having sort of blacklists of tenants that complain, or that are likely to bring up issues that's really, really concerning and would definitely deter some tenants from speaking out."
Real Estate Institute property management head Joanne Ray was adamant landlords were making progress.
"Most of the property management organisations that we deal with our reporting back that they're at least 70 percent of the way through, so there's been a real focus on completing the work."
In two years, landlords that do not meet their obligations under the healthy homes standards will be in breach of the Residential Tenancies Act.
They may be liable for damages of up to $7200, but only if tenants take it to the tribunal.
