New Zealand had an average household size of about 2.6, one of the highest in the OECD, he said.
This was likely to reduce to around two people per household, placing huge strain on the housing market.
"We all say that we've got a massive housing shortfall, we all seem to believe that it's got a lot to do with immigration, but nobody talks about the giant elephant in the room which is changing household sizes. And I think in the future this will be a massive problem."
Hartwich warned that Auckland house prices could reach the stratospheric levels of Sydney if at least 113,800 more houses weren't built within 20 years.
In April, average sale prices for houses in Auckland reached $700,000. According to one recent study, the median Sydney house price was $900,000.
Labour leader David Cunliffe said his party planned to block non-residents from buying property, and a capital gains tax to limit "rollercoaster" house prices.
Housing Minister Nick Smith said Labour's policy was designed to "scratch an itch, whereas I'm focused on addressing the long-term change that's required to improve home ownership and housing affordability."
Smith said the findings of the Productivity Commission would form the basis of his new housing policy. Its report, released in April 2012, said new land should be immediately released for housing development in "high-demand areas" such as Auckland and Christchurch.
The minister said the Government would continue addressing problems of land supply, council development charges and even the cost of construction materials.