The composer's latest orchestral outing finds him once more in the war zone, but this time the conflict is that of the 1914-18 Great War.
Williams relishes working on a more extensive canvas, defining the symphonic form as "a large-scale orchestral work made up of contrasting movements or parts". It is very good that symphonies are there, he says, "otherwise everything would have to be scaled down".
There are very personal connections in this symphonic debut, as the composer's great-grandfather, Arthur Major, lost his life at Passchendaele in 1917 and extracts from Major's letters will be read by actor George Henare, as an integral part of the new score.
Williams treasures the memento of his great-grandfather's wallet, with a picture of his family, which he carried with him in the trenches. "When it was returned they hadn't even bothered to clean off the blood and skin. He must have been shot through the heart and the bullet went right through his wallet."
Major's voice is not the only one you will hear on Thursday. As part of his research for this undertaking, Williams found wartime letters from soldiers of other nations, some as far afield as Russia.
"They were all written from the front to families back home, and the sentiments expressed are very much the same regardless of country."
The other significant component in Williams' score is provided by soprano Madeleine Pierard, who sings a setting of Wilfred Owen's Arms and the Boy with its striking images of "the famishing of flesh" and "blind blunt bullet-leads which long to nuzzle in the hearts of lads".
Some listeners may be surprised when, early on, Pierard delivers a crucial line in Latin. It is a translation of a line taken from one of the letters written home from the front: "My heart is so shattered I don't know whether it is broken or not."
"I often use Latin in my works when I'm trying to express something that is global or universal," Williams says. "A sentiment that rings true throughout the world."
Williams is the kind of composer who cheerfully tells you how he always likes to have "a big project on the go". Running at 35 minutes, the new symphony may not be his longest score to date but he says "it took more out of me than anything else I've written, through its size rather than its length".
Best of all, the Spirit of Anzac programme will play on both sides of the Tasman next week.
The NZSO and Sydney Symphony Orchestra both give two performances of a concert that NZSO chief executive Christopher Blake describes as "a musical moment for reflection through which we can mark this significant centenary".
Performance
What: New Zealand Symphony Orchestra, Spirit of Anzac
Where and when: Auckland Town Hall, Thursday at 7pm