As a responsible Government agency, the NZ Transport Agency's job is to thoroughly investigate the case for Light Rail between the city centre and Mangere. While some people are speculating about what light rail may or may not be, we are getting on with the job.

We've been drawing on the expertise of transport and urban development professionals both here in New Zealand and overseas. This includes the likes of Volterra from the United Kingdom, who have provided economic analysis on light rail projects from London to Kuala Lumpur to Adelaide. Systra, a transport consulting firm based in France, has been a part of the development of more than 40 light rail lines worldwide. We're also working with LUTI, who've worked on light rail across Australia.

Auckland Light Rail has much more ambitious goals than simply moving people between the city and the airport. They are light rail's bookends, what happens in between is the key reason we're building it.

The preferred route between the City Centre and Māngere has been developed after comprehensive studies based on two central goals - providing better transport access and more capacity to support growth in the CBD and wider urban areas.

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The City Centre and Māngere, including the airport, represent two of Auckland's fastest growing employment centres. More than a third of Auckland's job growth over the next 30 years is expected to take place in the City Centre to Māngere corridor. The communities along the route are currently home to nearly 200,000 Aucklanders and in the next 30 years, the population will grow to an estimated 300,000 people.

They are communities that have large areas of publicly owned land that give us a rare and important opportunity for large-scale redevelopment close to the city centre and the airport.

Light rail will provide greater transport access to suburbs that don't currently have good public transport options and this will in turn attract private and public investment in new housing developments. It will also connect with walking, biking and other public transport options, giving people much greater choice than they've ever had about how they travel.

In the City Centre, light rail will take passengers off buses, which are already adding to congestion and is forecast to get worse. That congestion is limiting the ability of the city centre to grow and deliver higher economic growth and productivity for Auckland and New Zealand.

Heavy rail is a much more expensive option and won't deliver the same urban development opportunities that light rail does. Studies show heavy rail between the airport and Onehunga would cost approximately $2.3-$3billion, and the airport to Puhinui between $1.7b and 2b. Either of these options would serve tens of thousands fewer people who live and work along the light rail route and it wouldn't ease traffic in the CBD or along the corridor.

Heavy rail would need a rail tunnel through to the airport that would be as long as the City Rail Link, that's a costly and time consuming project in its own right. Light rail can be built on existing corridors, making it faster and more cost effective to build and that means people will start enjoying the benefits sooner.

These maybe the very reasons that in May 2015, Councillor Mike Lee, who is now campaigning for heavy rail, was in fact celebrating light rail as the best news for Auckland's transport since rail electrification.

While opening up more communities is at the heart of this project we also know access to the airport is important. That's why we're working together with Auckland Transport and Auckland Airport to improve those connections through the Southwest Gateway programme. This includes an upgraded station at Puhinui to connect new bus services with heavy rail and also includes development of a rapid transit link between Botany and the airport.

I am proud the NZ Transport Agency is at the forefront of building a rapid transit network to transform how people get around Auckland. We've proved we can do it well, look at the hugely successful Northern Express Busway. Light rail will be another huge step forward in creating a world class public transport system, giving people better access to jobs, education and recreation and making Auckland a better places to live.

* Fergus Gammie is chief executive of the NZ Transport Agency.