Landlords faced added pressure to complete the works because - from July 1 - all rentals needed to meet the insulation, heating and ventilation standards within 90 days of a new tenant moving in.
Betta Group chief executive Matt Mason said finding the cash and time to fix up the rentals could be hard on landlords.
"But ultimately, if we get warmer, drier houses and it improves health and lowers the cost on our medical profession, it has to be a good thing," he said.
The Government brought in the laws aiming to help the estimated 1.4 million Kiwi renters living in 527,853 rentals to lead healthier, more stable lives - but reactions have been mixed.
Health groups support the laws, saying cold and damp houses play a part in causing asthma, rheumatic fever and respiratory infections - commonly known as respiratory diseases.
Respiratory diseases affected 700,000 Kiwis and were responsible for almost 80,000 hospital admissions at a cost of $6 billion each year, the Asthma and Respiratory Foundation said.
Landlords often supported the idea of healthier rentals but have quibbled over some technical details, claiming, for instance, the Healthy Homes laws required them to install more powerful heaters than were often needed.
Many also said complying with the Healthy Homes laws added to their costs at a time when the Government was already targeting property investors with tax changes and other measures, partly aimed at discouraging them from buying more rentals.
Landlord Ben Curran called the cost of Healthy Homes compliance a "massive kick in the teeth".
He earlier had to top up his two-bedroom unit's insulation, despite a building report at the time he purchased the property, stating it was up to standard.
"I've actually lived in the house myself for a period of time with my family, and we had no dramas keeping the place warm," he said.
He said he had "sacrificed a lot, including living overseas away from family" to try to invest in Auckland's housing market for his future, and it seemed unfair to now be forced to foot unnecessary costs.
Landlord Mike McDonald, who spent $2500 installing a heat pump, said the new standards were reasonable.
He was renting out his family home while he lived and worked overseas and that meant the improvements would one day directly benefit him when he moved back in, he said.
Mike Atkinson, managing director of Auckland-based Aspire Property Management, said he was working on behalf of 430 landlord clients to inspect and get their properties up to standard.
He said his team was steadily working through the inspections and upgrades rather than rushing.
That was because landlords had to comply with the changes only if their tenant moved out.
If the same tenant stayed in their property, the owner could wait until 2024 to make the changes.
Betta Group's Mason said the most common way rentals were failing was from draughts.
These were often gaps, 3mm or bigger around timber windows, that allowed air to blow in from outside, with only 46 per cent of homes inspected by Betta Group passing on the first go.
Just 67 per cent of rentals passed the "drainage and guttering" category, with those that failed, doing so because water was not properly draining from the property, such as off the roof into a stormwater system.
Heating with a 74 per cent pass rate and the installation of a moisture barrier - a barrier laid under the floor to stop rising damp - with a 76 per cent pass rate were the next most common failures.
The Green Building Council, meanwhile, had been running an online tool called HomeFit to help homeowners check if their houses met Healthy Homes standards.
The council said that based on 20,000 checks of its tool by net users, up to 90 per cent of rentals would fail an inspection.
Council chief executive Andrew Eagles said Healthy Homes was important because better rentals could keep families warm, children out of hospital and slash carbon emissions - in turn delivering a big boost to the economy.
But he worried the Government didn't have the resources to make sure landlords complied with the standards.
The Ministry of Business, Innovation and Employment said it could do 2000 risk-based interventions each year to check on compliance, yet New Zealand had hundreds of thousands of rentals, Eagles said.
"The Government hasn't geared up enough to check these standards are being met," he said.
