The terrible destruction of the centre of Christchurch has been a tragedy for the citizens, not only the loss of life and property but also the loss of New Zealand's best planned city - a garden city on the plains with a clearly defined centre of cathedral and square. The contrast to Auckland could not be clearer. The European founders of Auckland actually had a plan, drawn by Felton Mathews in 1840, but they ripped it up almost immediately and the city has been growing where ever it wanted since then.

What does unify these two very different cities is the emptying out of the city centre. Even before the earthquakes the Christchurch CBD was being slowly abandoned, as urban development moved to the surrounding suburbs and region. Auckland's CBD lost its importance long ago, with only 13 per cent of the population working in the inner city.

This is not just a New Zealand problem, but a worldwide phenomenon. The causes are complex but increasing affluence, the massive proliferation of private transport infrastructure and a revolution in communication technology, have all contributed to the end of the traditional city centre. In Christchurch the physical destruction of the CBD has been compounded by the realisation that it will be extravagantly expensive to build and hard to insure any future construction in the CBD.

This flight from the CBD has often been presented as a terrible problem. But what if it was actually a tremendous opportunity? The world is in a desperate search for solutions to the looming global ecological disaster. The biggest culprit seems to be the city.

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Cities consume three quarters of all planetary resources and generate more than three quarters of all waste. And they are growing -- more than half of humanity is already urban. In about a decade we will have about four billion people in urban areas, and in two decades, about five.

Ensuring that these staggering numbers of people have such basics as food, water, energy, shelter and sanitation is the most daunting task that humanity has faced. And the task will only get harder under the conditions of unstable climate and more frequent extreme weather events.

We need an urban technological revolution. It will consist of a profound change in the design of urban technical systems. They will have to be clean, green, small, smart and decentralised. Think of the new city as a giant farm - a sprawling artificial landscape that harvests clean energy, food, rainwater and treats and recycles its waste locally.

Think of it also as a mix of low-tech and high-tech solutions, as sophisticated ICT solutions are deployed along with clean-tech to make everything super-efficient and stimulate more responsible human behaviour. This is where New Zealand can lead the world.

Christchurch was founded as the 'garden city' - an ideal place to live. Auckland aspires to become the 'most liveable city in the world'. Both cities owe their lifestyle to being close to nature. They do this simply by being low density and thus allowing nature's presence almost everywhere - in the form of our gardens, parks, reserves, and beaches.

What is the problem with low-density cities? Critics believe that low density equals urban sprawl. However, with strict protections in place for: areas of exceptionally fertile soil; vital watershed drainage; sensitive and/or rare habitat; attractive landscape, and the adoption of clean and autonomous urban infrastructure, low-density cities can assume a coherent urban form. Such a form includes many nodes of denser development, with services such as public transport, and facilities for a vibrant social life.

Indeed, NZ is uniquely poised in this moment in history. No other country has its two main cities both under major re-examination - one, sadly, due to a disaster, the other due to political transformation. The world urgently needs a template for a viable, safe, low-density city. NZ could not only provide the answer, but also provide it in an attractive - as well as affordable - form.

In the process, NZ could discover gold; its industries would be embarking on a form of green knowledge economy that is certain to be the greatest growth stimulus in the 21st century and a big exports booster. Rebuilding Christchurch, as it was - based on a centric, compact layout, supported by traditional, expensive and vulnerable infrastructure - and continuing to grow Auckland in that fashion too, while fencing it off from one of the best residential landscapes in the world - would be an environmental, cultural and economic tragedy.

* Dr Dushko Bogunovich is the Associate Professor of Urban Design at the Unitec Institute of Technology and Matthew Bradbury, Fulbright Senior Scholar, is a Senior Lecturer in the Landscape Architecture programme at the Unitec Institute of Technology.