A proposal to use vacant houses in Auckland for the homeless or lower-income workers is being discussed by Auckland Council and the Government.
But it could be complicated by privacy concerns, because the plan would require utilities companies to find and contact the owners of homes which are not using any power or water.
The latest Census showed that the number of vacant properties in Auckland had risen from 6.6 per cent to 7.3 per cent of the city's private dwellings - around 40,000 properties.
A large number of these were holiday homes or properties which were temporarily vacant. But a portion of them were "ghost houses" - deliberately left vacant long-term by investors who wanted capital gains without having to manage tenants.
Auckland Mayor Phil Goff said he had spoken to ministers about finding a way to utilise these homes, and they had shown an interest.
"You could get utilisation of several thousand homes around Auckland that are just being left empty - maybe by developers who are thinking 'We are just going to develop this place in 12 months' time'.
"If we can make it easier for them to utilise that house that would obviously be a win-win for landlords, prospective tenants and for Government and council and tackling the housing crisis."
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Goff said electricity company Vector could find out what houses have not used power for more than six months.
"So we can get accurate information, but we would need to find a way to ensure we were not in breach of privacy laws in doing that.
"Then the company themselves could write a letter to the property owner and said 'We noticed your house isn't being used at the moment', would you consider having it managed by Housing NZ or someone else?"
The council and NGOs say vacant houses could potentially be used for the Housing First programme, which finds shelter for homeless people. They could also be an option for middle-income workers like teachers, nurses, and police officers who were struggling to find affordable accommodation in the city.
In 2015, Vector found around 8000 homes in Auckland - or 1.6 per cent of all dwellings at the time - were unoccupied, meaning they had used less than 400W of power a day for 100 days or more.
Most of them were in northern beach suburbs and Waiheke Island, which led the company to conclude they were holiday homes and baches.
While Goff believed that Watercare would also be able provide data on vacant houses, the organisation said this was not possible.
A spokeswoman said that its meters were not set up to measure zero water use.
Any research would be complicated by blocks of flats with shared water meters or single properties with multiple meters, she said.
Many of the vacant properties are believed to be "lock and leave" apartments, which owners purchase and leave empty to cash in on capital gains. But there are also standalone homes and luxury mansions dotted around the city which are being left vacant.
On an expensive, popular street in Ponsonby, one villa has been vacant for 10 years.
A neighbour, who did not want to be named, said the landlord used to rent the property but had decided it was easier to leave it empty.
"He just didn't want the hassle - of filling the place, doing the maintenance. The lawns are still mowed, but it never has tenants."
In the time the home has been empty, it has risen in value from $700,000 to $1.52m.