Thousands of Kiwi children are homeless, with many missing out on schooling and basic medical care.
A new Otago University study has used 2006 census data to provide the first measure of homelessness, finding that 34,000 people suffer "severe housing deprivation".
Researchers say the problem has "inevitably" worsened since 2006, with a deepening housing shortage and rising demand for support services.
Far from the stereotype of the grizzled man sleeping on the street, more than half of New Zealand's homeless were under 25, and a quarter were children. Most lived temporarily with friends or family, squeezed into living-rooms or garages, rather than on the streets.
Children's Commissioner Russell Wills said an alarming number of children had "no fixed abode".
At many low-decile schools, this could lead to a yearly student turnover of more than 50% as families flitted from house to house.
These children usually did not have a doctor and could spend long periods not attending school as they moved, he said.
"These are low-income working families that can only get accommodation in a garage or in a shed. It leads to terrible outcomes for children."
And while homeless people were more likely to be unemployed, the study shows about half were working or studying. About one in five were unable to afford a home, despite working fulltime.
Otago University researcher Kate Amore, who headed the study, said New Zealand did not keep a good watch on its homeless and had only recently defined the term.
"We identified many severely deprived people who are usually statistically invisible because they are not living in permanent private dwellings," she said.
She estimated that between 12,000 and 21,000 extra affordable homes were needed to accommodate the homeless.
Nearly half the homeless lived in Auckland but the figures show that in 2006 there were 2627 in the Wellington region, including 945 in the capital.
Stephanie McIntyre, director of Wellington's Downtown Community Ministry, said the scale was not surprising. "But it is still quite disturbing."
Homelessness was inextricably linked to a shortage of affordable housing, which had worsened since 2006, McIntyre said.
In the past five years, the number of clients visiting the Downtown Community Ministry each year had doubled to more than 800.
"For an agency like us, we can't do our job adequately without more affordable homes. Yes, people need support but the fundamental reality is people need somewhere to live."
The Government made several rule and funding changes in relation to affordable housing in this year's Budget.
This included upgrading existing Housing New Zealand homes, shifting needs assessments to the Ministry of Social Development, and opening up funding for non-government organisations to build more affordable homes.
The study was conducted by Otago University's housing and health research programme, and was funded by Statistics New Zealand and Housing New Zealand.
Who is homeless?
Statistics New Zealand defines a homeless person as one who does not have a secure, safe, habitable and private place to live.
This includes not just people living on the streets, but those in temporary accommodation, such as a boarding house or a camping ground, or sharing housing, such as sleeping on friends' couches. Living in an uninhabitable house, without water or power, also counts as homelessness.
Until 2009, there was no definition of who was homeless in New Zealand and there are still no reliable regular national figures.