There are calls across the ditch for a vacancy tax on empty houses to improve affordability.

There's an estimated 25,000 unoccupied dwellings in Melbourne alone and it looks to be a similar situation here.

According to the latest census data there are more than 185,000 empty houses across New Zealand.

At the 2013 census more than 22,000 houses in Auckland were unoccupied.

Property Institute chief executive Ashley Church told Mike Hosking the tax is a bad idea and questions how "empty" would be defined.

LISTEN ABOVE: Ashley Church talks to Mike Hosking

"Are you talking Mum and Dad own a home that's three bedrooms and you tax the other two rooms? Are you taking about people that have the audacity of having a bach or holiday home at the beach? There's a whole range of unintended consequences that would kick in as a result of something like this."

A group in Melbourne's proposing property owners who leave their homes empty for more than 12 months face a tax.

It says people who do this are worsening housing crises, and should pay for the social harm it causes.

Mr Church said he understands the motivation behind the tax.

"But it would be an extraordinarily draconian move in a free-market society. Where would you stop? Would you tax people who don't use their cars on certain days?

"The solution to that problem is to improve the supply of houses as quickly as possible by building more homes, not to try to penalise people who, for whatever reason, are utilising that stock as they choose to within their free-society rights."

President of the home owners and buyers association of New Zealand John Grey thinks it's overseas property investors who are responsible for the unoccupied houses.

"There'd be very few New Zealand investment property owners, who actually have homes that are empty, other than for situations whereby they are perhaps in need of significant repair."

CEO of charity Launch Housing Tony Keenan said many people are left homeless, or are in temporary accommodation, because of these empty houses.

He said the tax could do something to fix that.

"It's levied on people who choose to leave houses empty. If people rent them out they don't have to pay the cash, but if they choose to leave them empty, that money goes towards social housing."

Keenan said each time a landlord decides to keep a house empty, they're worsening the housing crisis.

"We're saying that when they make that choice, they then make a contribution to the social harm they're causing in not making those houses available."