The couple then set about contacting all the Youdales they could find in the Sydney suburb where he was born, until they finally found a great-nephew who thought the medal was in the Imperial War Museum in London.
Even the Roberts' sleuthing abilities have not been able to solve the mystery of how the medal ended up in Russell, a town with which Alfred Youdale had no known connection.
On Monday, Armistice Day, four members of Alfred Youdale's family flew from Sydney to attend a formal unveiling of his Military Cross with Two Bars and accompanying service medals in a purpose-built display case at the Russell RSA.
An emotional Mr Roberts said there were two tragedies to Alfred Youdale's story.
''First, that Alf was killed in action on December 23, 1917. Second, he never got to wear these medals.''
Great-nephew Philip Youdale said he and his father, who was about to turn 90, were ''incredibly surprised'' to learn the medals had surfaced in New Zealand.
He had never met his great-uncle but his exploits and derring-do were well known in the family, thanks to his prolific diary and letter writing.
He had somehow survived eight months in Gallipoli before falling ill and being invalided home to Australia. As soon as he recovered he returned to the war, this time training as a pilot for the Royal Flying Corps. His training totalled one hour's flying a day for six days.
He gained such a reputation that people would gather at the aerodrome to await his return and hear his latest exploits. He survived being shot down eight times. The ninth, somewhere over France or Belgium, proved fatal.
Philip Youdale said he had spent the past four months re-reading the diaries and putting himself in his great-uncle's shoes.
The family was proud his medals were on display in Russell, helping to keep his story alive, instead of being hidden away in a drawer.
Among those at the ceremony and Armistice Day service were representatives of the British and Australian High Commissions, MP Mike Sabin and 90-year-old WWII Bomber Command veteran Ray Tait, who unveiled the medals. Children from Russell School also took part.
The Youdale family has been given a set of replica medals to take home.
... but one mystery remains
How Alfred Youdale's medals came to be in Russell is a mystery that may never be solved.
Jack Hayes, advisor of medals policy for the New Zealand Defence Force and trustee of the United Services Medal Collection Trust, said the Military Cross with Two Bars was very unusual. Only nine were awarded worldwide in WWI and Russell's was the only known example in New Zealand.
''One of the mysteries is we still don't how it ended up in Russell,'' Mr Hayes said.
One of the theories was that one of Alfred Youdale's five sisters may have brought it to Northland and a later generation, unaware of its significance, had given it to RSA. There was no record of who had donated it or when.
The family had been convinced it was in a museum in London but that was an unengraved duplicate. When a soldier died in WWI it was common to have duplicates made, especially of gallantry awards, for family members.
Mr Hayes said Monday's ceremony was significant because it was a tri-nation service. Alfred Youdale was Australian by birth, joined the Royal Flying Corps (then a branch of the British Army), and his medals had ended up in New Zealand.
With the blessing of the family the RSA had gifted Alfred Youdale's medals to the United Services Medal Collection Trust, which meant they were always protected - including from receivers if the RSA were ever to go broke. The Trust had then loaned them permanently to the Russell RSA where they would remain on display.
They would be loaned to the Returned Services League in New South Wales for six months so other family members could see them. The Trust deed specifies that its medals can never be sold or given away.
