Prince William and Princess Astrid of Belgium unveiled a new commemorative plaque at the centenary of the Battle of Passchendaele, with the Prince saying the strength of character New Zealanders showed is still admired today.
It commemorates the "enduring bonds of friendship" between Belgium and New Zealand.
On October 12, 1917, New Zealand suffered its greatest loss of life in a single day, as some 960 soldiers were either killed or mortally wounded at the Battle of Passchendaele, near Ypres, Belgium.
Another 2000 were wounded or taken prisoner.
Prince William said the soldiers of WWI were often described as ordinary men, fathers, sons and brothers called on for their duty.
"There was nothing ordinary about their service or sacrifice," the Duke said.
William said he'd seen how the deadly day had marked itself on the national consciousness when he visited memorials at Blenheim and Cambridge with wife Catherine several years ago.
"Ka maumahara tonu tatou ki a ratou. We will remember them," the Duke said.
Princess Astrid thanked New Zealanders for their sacrifice when her country was destroyed by war.
The royals arrived at the ceremony flanked by the New Zealand Defence Force cultural group.
Speaker of the House David Carter and NZ Defence boss Lieutenant General Tim Keating also paid tribute during the service.
During the month the New Zealand Division was engaged at Passchendaele some 1900 soldiers died, another 4100 were wounded or evacuated sick.
The Tyne Cot Cemetery is the largest Commonwealth War Graves Cemetery in the world, with 11,000 buried here. The memorial is dedicated to 35,000 soldiers who died in the Ypres region after August 1917, whose graves are unknown.
There are 198 named New Zealand graves at Tyne Cot Cemetery and 322 unknown graves. The memorial commemorates 1166 New Zealanders.