Ratepayers will stump up an extra $200 each a year to pay for Auckland to build past the city limits, the Greens say.
The Auckland Plan committee agreed last month to look at undeveloped land outside city boundaries for up to 40 per cent of new housing over the next 30 years.
Green Party Auckland spokeswoman Denise Roche is calling for all new housing to be based around transport hubs or within city limits.
She believes ratepayers will recoil when they realise how much greenfields development will cost.
Developing within the city will save ratepayers massive bills for new roads, stormwater, sewerage and water reticulation, she says.
"I don't mind paying more for things we need. But we need to be sensible about what we can afford to do."
Green Party researchers calculate the council's plan will cost ratepayers $42 billion to 2042, an annual levy of $200 per ratepayer, Roche says.
Turning greenfields into housing also removes food-growing land, she says.
The Green Party calculations are based on a 2009 Australian study by Curtin University.
The study found sprawl development costs A$19,800 more for every new house.
The study also found new infrastructure was only part of the cost - the affect of increased pollution and health problems were also passed on to the public.
Auckland University school of architecture and planning senior lecturer Patricia Austin agrees building outside city limits costs more, but does not know how much more as no study exists for New Zealand. One is needed, she says.
"There's definitely going to be costs. How big they will be for Auckland, we don't know."
Increased health costs are a good example of a hidden cost.
"There's a mass of information on what the Americans call obeseogenic environments.
"You have to get in a car to go anywhere, so you get fatter and you get sicker, roughly speaking."
Auckland Council chief planning officer Roger Blakeley says the council knows greenfields development is more expensive than building in existing urban areas.
For that reason, 60 to 70 per cent of growth projected in the Auckland Plan will be inside existing boundaries, partly through higher density, so the city can grow while minimising the cost.By Cherie Howie Email Cherie