Auckland has lost 4 per cent of its most fertile rural land to housing over the past two decades and is set to lose more under the strain of a million more residents.
High-class agricultural land at Henderson, Massey, Greenhithe, Albany, East Tamaki, Takanini, Weymouth and Papakura has been lost to urban sprawl, according to Landcare Research.
It found that successive waves of urban growth between 1990 and 2008 resulted in Auckland losing 4.1 per cent of the available high-class agricultural land to urbanisation and 35 per cent to lifestyle blocks.
Landcare Research is cautioning the Auckland Council that when it comes to opening up more rural land for housing, not all rural land is equal.
Some parts of the rural land where the soils and climate support thriving produce (for example, the rich soils around Pukekohe), dairying and bloodstock sectors, are more valuable.
Urbanisation of agricultural land was essentially an irreversible process, said Landcare researchers John Dymond and Robbie Andrew.
Dividing the land into smaller parcels for housing produced a sharp rise in land value, made it difficult for farmers to buy it back and could take centuries to return to its initial state.
They believe New Zealand needs to give more thought to the value and protection of high quality soils and productive land, particularly given ongoing worldwide issues around the supply of food. Auckland Council land and soil scientist Dr Fiona Curran-Cournane said spatial analysis showed that about 10,400ha of elite and prime land had been lost to urbanisation between 1996 and 2010.
She said soils were a non-renewable, capital asset that once developed were lost forever through irreversible damage and degradation.
"Some of the best, most productive soils in New Zealand are located in South Auckland, supporting a significant proportion of New Zealand's outdoor vegetable production," Dr Curran-Cournane said.
The Auckland Plan - a blueprint for the city and basis for the Unitary Plan - has a target of no more than 10 per cent of rural subdivisions to be on rural productive, coastal and active island land.
The draft Unitary Plan has provided for 160,000 new homes outside the existing urban boundary in a new boundary called the "rural urban boundary".
About 90,000 of these homes will be on new land at Warkworth, Silverdale, Whenuapai, Kumeu and Huapai in the north/northwest and Pukekohe, Paerata, Drury and Karaka in the south.
The other 70,000 homes will be within the existing big towns of Warkworth and Pukekohe, and rural and coastal towns and villages such as Beachlands, Bombay, Snells Beach, Piha, Wellsford and Whitford. The 70,000 figure includes developments under way outside the urban limit at Hobsonville and Hingaia. The 160,000 homes are all outside the existing "metropolitan urban limit"- an urban boundary set up in the late 1990s to contain urban sprawl.
The housing mix in the draft Unitary Plan is 60 per cent to 70 per cent urban and 30 per cent to 40 per cent rural.
Council chief planning officer Dr Roger Blakeley said the work of opening up rural land for housing sought to protect all of the class 1 highly productive elite land, while avoiding, where possible, prime, class 2 land.
The Landcare Research work is at odds with a report by policy thinktank the NZ Initiative which says fears Auckland will sprawl over farmland are exaggerated and opening up more rural areas for housing would ease house price pressures.
The thinktank report, How New Zealand Lost Its Housing Affordability, said less than 1 per cent of New Zealand is built on "even after including landfill and roads". It did not give a figure for Auckland, which is about 10 per cent urban land.
The report said Auckland's rural urban boundary had been almost completely shut for 10 years, causing rising land values, favouring the old and rich and punishing the younger and poor.
Hugh Pavletich, co-author of the annual Demographia Housing Affordability Survey, said that New Zealand, which has about the same land area as the United Kingdom with a population of 63 million, had only 0.7 per cent of its land urbanised.
"There is no need to constrain our cities. We couldn't urbanise a further half a per cent of our land area over the next 50 years even if we met all the demand for all the housing that's needed," he said.
Housing Minister Nick Smith also favours Auckland breaking out of its boundaries, saying putting a straitjacket around the city for residential development only benefited land bankers. "We cannot walk away from the issue that restrictive land supply policies across the world are at the heart of the housing affordability issue," Dr Smith said.
Federated Farmers chief executive Conor English, writing in the Herald earlier this year, said Auckland had to stop gobbling up productive land having already lost large swathes over the past 30 years to urban sprawl.
He said instead of spreading all the way to Taumarunui, Auckland needed less traffic congestion, more public transport, better utilisation of resources, and more integrated and diverse communities. "To do this it simply has to go up, not out," said Mr English, whose brother Bill English is Finance Minister and is taking a keen interest in measures to take pressure off runaway house prices.