Mouldy and damp homes are huge problems for renters while landlords place little importance on heating and ventilation, a survey says.
The online survey asked 1040 people, tenants and landlords, about renting and letting.
It found more than half the rental properties surveyed had mould around windows and in the bathroom.
Auckland University Professor of Sociology Charles Crothers said fixing damage was a priority for most landlords, but only a small number - four percent of the survey respondents - thought mould and dampness were important.
"They've got a bit of a way to go, in terms of the investment that they should be putting into things like insulation, double-glazing and making it easier to keep warm," he said.
Professor Crothers said dampness and cold increased renters' heating costs, worsened their health, and ruined furnishings.
He said about two-thirds of tenants said their landlord did not respond or failed to properly fix a problem.
Central Wellington tenant Annie Dick said her flat had problem after problem.
"Leaking roof, mould, rats in the house, broken windows that they wouldn't fix, we're paying extremely high rates for a property that's falling apart, it's just ridiculous," she said.
Ms Dick said she and her flatmates have all already had colds three times this year.
Dunedin student Lily Whitworth said she lived in a flat where the mould was so bad a mushroom sprouted from a windowsill.
She said her landlord did not do anything.
"He came and had a look at it, plucked it out of the wall and said, 'Oh, I thought I told you, you weren't allowed to have pets'."
A central Auckland landlord Neeraj said he did his best to keep his properties safe and warm, but not all landlords did so.
"I know some people who take a very hands off approach or [are] very lazy in some aspects, especially when it comes to spending money.
"They're just not willing to look after their tenants."
The survey was conducted by research firm Buzz Channel with Auckland University of Technology for heating and ventilation company HRV.