And now the elusive manuscript of the piece that, until recently, existed only as references in diaries and letters, forms part of the National Library's collection in Canberra.

It joins the extensive papers of the Australian-born composer and war hero Frederick Septimus Kelly, who survived two woundings at Gallipoli and was awarded for gallantry during the evacuation in January 1916, only to be killed in action in the Battle of the Somme in November of that year.

The manuscript arrived in the collection last week, just in time for the commemorations of the Anzac centenary on April 25.

Senior curator Robyn Holmes said researchers had always known Kelly had composed the sonata shortly after writing an elegy to his great friend, the poet Rupert Brooke, who died at Gallipoli, and that he had deliberately wanted to offset its sombre tone.

He wrote the piece expressly for the Hungarian violinist Jelly d'Aranyi, telling her, during the composition process, "You must not expect shell and rifle fire in it! It is rather a contrast to all that, being somewhat idyllic."

"It's almost like this is the survival mechanism of an artist on the battlefield," Ms Holmes said.

"It might not represent what's happening on the battlefield, it's rather a way out, imagining and envisioning life outside that."

It was Canberran music director and scholar Chris Latham who first tracked down the manuscript, having traced d'Aranyi's last descendant, a great-niece, in Florence in 2010.

Although the relative allowed Mr Latham to make a copy of the work so that he could play it in Australia, he said she had long struggled to know what to do with d'Aranyi's papers.

"I told her that really, the best place for the manuscript was here in Australia with the rest of Kelly's things," she said.

The relative eventually agreed, and the library acquired the manuscript this year.

Ms Holmes said it a vital last piece of the Kelly puzzle. In 2013 the library acquired his last known handwritten diary, dating from February to April, 1916.

"What we discovered on the end of the Gallipoli sonata was the date that he actually copied out the score," she said.

"He composed it during Gallipoli's fighting in his head, but he hadn't actually notated it or written it down properly. And on the score, it's dated February 28 and 29, 1916, and it was copied out on the HMC Olympic, so he's on the ship on the Mediterranean sea."

Curators were then able to check the diary itself, and found an entry on the same day mentioning that he had written down the score.

Mr Latham said the piece itself was startling in its contrast with the horror that surrounded Kelly while he was writing.

"It recalls to a degree Nelson Mandela in Robben Island prison," he said.

"Kelly displays complete freedom to think and create regardless of his external circumstances. He is able to imagine an unusually clear, coherent and beautiful sustained idea within a state of real danger and chaos."

Ms Holmes said the Kelly collection would still be significant in 100 years' time.

"It's also a story about what I call the sustaining spirit, the sustaining life of the artistic spirit in shocking war duress and how important music is to understand," she said.