His appointment, in December, 2012, coincided with planning to meet one of the biggest challenges the memorial had ever faced; planning for the appropriate national commemoration of the centenary of Australia's involvement in World War I.

This was, after all, the conflict that had given birth to the push for a national memorial in the first place. It was also, as Dr Nelson has noted on numerous occasions, a defining event in our national journey which bought Australians together in a way the act of Federation could never have done alone.

The AWM was not just commemorating a global conflict; it was acknowledging the birth of a nation. And, as Dr Nelson has also said, the war was a tragic period in our history which united tens of thousands of Australian families in grief and heartbreak.

Anybody who has ever spent time with the director in the Hall of Memory or pacing the Roll of Honour was immediately struck by the way this doctor turned politician, turned diplomat, turned institutional head could bring history to life.

He has never made any secret of his belief that while the memorial is a shrine, a museum and a centre for research and academic excellence, its core function is to honour the more than 100,000 Australian men and women who gave their tomorrows for our today.

While Dr Nelson has been criticised for his passionate advocacy, based on a sincere belief it would be impossible to do too much in this regard, those who have heard him flesh out some of the remarkable stories behind the names on those walls would find it hard to disagree with him to his face.

Despite not being a trained historian or a professional soldier, he has brought unique and valuable gifts to his role as the keeper of the place where, in his words, the soul of the nation resides. These include an abiding passion for Australian social history that complements an interest in all things military and a strong empathy with the millions who have come to honour the fallen and to better understand what they experienced on his watch.

The memorial is a shrine to the rawest and most powerful emotions Australians have ever experienced. These include love, a profound sense of mateship, self-sacrifice and an abiding grief that was, for many, to end only with death.

Dr Nelson's particular talent has been to use his own humanity, often fleshed out by his own forbears' experiences of war, to bring this emotional powerhouse to life.

That humanity also extended to a determination to make sure the 100,000 new veterans created since the turn of the century by conflicts in Afghanistan and the Middle East, were not ignored in the same way the Vietnam veterans were a generation ago.

That led to his successful campaign for $500 million in Federal funding to enlarge the memorial's gallery space. It is, despite the ongoing debates over the way in which this should be done, a great economic shot in the arm for this city.

Dr Nelson's other gift, visible in his frequent visits to the galleries where he took the institution's pulse by talking to the visitors, was a recognition the AWM is a national institution, not just a Canberra one. He knew it has to connect with all of Australia.

As an administrator and as a leader it is fair to say Dr Nelson is leaving the memorial in better shape than he found it. There is a renewed sense of purpose and a clear road map for how the next decade should unfold.

The next challenge is to find a successor with the passion and the will to ensure this momentum is not lost.