The owner was Herr H. Held of Hanover.
Cullen's family had since tried to identify the wallet's owner since - to thank the owner or his family for effectively saving his life.
But it was only recently, in Belgium at the centenary of the Battle of Passchendaele, that the search turned up results.
Hayden Cullen was part of the Kiwi contingent travelling to Belgium to commemorate the 100-year anniversary of the devastating battle. Before leaving, he had posted a video on social media in a bid to find the wallet's owner.
"After exhausting all avenues from New Zealand, the family were really pinning their hopes on me finding a lead in Europe," Private Cullen said.
The post was picked up by local media, and German outlets sparked an international mission to track down the owner's family through the few address details written in the wallet.
Miraculously a German newspaper confirmed the German soldier's identity, as well as locating and speaking to the German soldier's great-niece.
It was a bittersweet meeting for Hayden Cullen, who found his great-grandfather's rescuer had died in 1929 without marrying and without children.
However Heinrich Held's great-niece Anja Rabe was thrilled to find out more about her late family member.
"I never knew much about Heinrich. He died before I was born and for whatever reason my mother never talked about him - and I regret not asking", she said.
Private Cullen and his family were delighted to put a name and a family to the wallet, and said it was great to thank Rabe for her great uncle's good deeds.