After the war the diary was kept in a secret drawer at Mr Langley's grandparents' home.
"Once my grandfather had shown me how to open that as a 6-year-old boy, that was my target," Mr Langley said.
He wasn't certain how his grandfather had come across the diary but it was probably either while he was on burial detail or in the trenches.
"From his point of view he obviously felt it was the right thing to do to keep it."
Mr Langley's grandfather died in 1967 and grandmother a few years later.
"During that time I really became interested in family history. I inherited the memorabilia that was in that secret drawer."
It was then the fragile and water-damaged diary began its journey to Wanganui.
"The challenge was to read as much as possible and make a transcript."
In recent years Mr Langley has taken it to handwriting experts and forensic labs to recover some of the unreadable text. He estimates about 85 per cent of it has been transcribed.
"At no point in the diary does (the author's) name appear."
Over the past four years Mr Langley has been in contact with staff at the Alexander Library, which houses the archives of the Wellington West Coast Regiment.
After trawling through archives and clues from the diary it is thought the likely author was Private Dorsett. Born in 1890, the Englishman emigrated to Palmerston North in 1910. He joined the regiment on October 16, 1914, and went with it to Egypt before heading on to Gallipoli.
The library's heritage services leader Gillian Tasker said she hoped people may come up with more information about who Charles Dorsett was.
A transcript of the diary will be made available, while the diary may be put on display after its condition has been assessed.
"For me, this ends a journey that started over 50 years ago," Mr Langley said yesterday. "In the end it probably doesn't matter who wrote this diary. My grandfather would be proud to know that the diary has finally come home."