Mould is lurking as a silent killer in more than one-third of all New Zealand homes despite the introduction of housing reforms to improve the health of our houses.

Statistics NZ's latest wellbeing survey found 41.5 per cent of Wellingtonians reported mould in their homes, while 34.9 per cent of Aucklanders reported mould.

Across the country, about 16 per cent of Kiwis reported seeing mould bigger than an A4 sized paper in their homes – the effects of which can be devastating.

The government's Healthy Homes Act will be introduced by July 2021, in a bid to address the issue.

City Sales' managing director Martin Dunn told Mike Hosking when the Building Industry Authority was formed under Helen Clark, unsuitable building materials were used.

"They allowed that hardy plank sort of crap to be used in building. The BIA allowed you to have no eaves, which looks very good but obviously leaks. And they allowed a deal with the Greens that allowed untreated timber."

He doubts that new homes will solve the crisis, as he says hundreds of homes in Pokeno are being built with unsuitable materials.

"This is an ongoing scandal in New Zealand and the aftereffects of it are just really appalling."

On average, Kiwis make about 28,000 visits to the hospital each year as a result of living in cold and damp houses, according to the University of Otago.