Prince William says there was nothing ordinary about the service and sacrifice of New Zealand soldiers who fought at Passchendaele 100 years ago.
On October 12, 1917, New Zealand suffered its greatest loss of life in a single day, as about 960 soldiers were killed or mortally wounded at the Battle of Passchendaele, near Ypres, Belgium.
Another 2000 were wounded or taken prisoner.
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The Duke of Cambridge, representing Queen Elizabeth, along with Belgium's Princess Astrid joined about 3000 people on Thursday at Tyne Cot Cemetery, the resting place of 512 New Zealanders.
Prince William said often the soldiers of WWI are described as ordinary men but "there was nothing ordinary about their service or sacrifice".
"Half a world away, news of the losses was felt like a shock wave - every death here left a shattered family there," William said. "No part of New Zealand was untouched by loss."
William said he had seen how what is now known as New Zealand's "darkest day" had marked itself on the national consciousness when he visited memorials at Blenheim and Cambridge with his 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. Prince William was greeted with a hongi from Willie Apiata, New Zealand's only living Victoria Cross recipient
Speaker of the House David Carter told the crowd how New Zealand soldiers described the battlefield as "a porridge of mud" and a place that "stamps itself on one's mind and memory like a red, hot iron".
"As inscriptions on New Zealand memorials across the Western Front read, 'They came from the uttermost ends of the earth'," Mr Carter said. "These soldiers were from places that could not be more distant from where we stand now."
A plaque inscribed with the Battle of Passchendaele is in parliament, Mr Carter said, and is a ready reminder of the solemn duty he and his colleagues undertake.
NZ Defence boss Lieutenant General Tim Keating also paid tribute during the service to the heavy weight of responsibility commanders felt on the Western Front, knowing they were sending their troops to injury and death.
During the month the New Zealand Division was engaged at Passchendaele, about 1900 soldiers died and another 4100 were wounded or evacuated sick.
The battle on October 12 proved to be particularly deadly for New Zealand, as they tried to capture Bellevue Spur, battling not only German forces but also the rain-soaked bog of what had once been rich farm fields.
The thick mud stopped New Zealand forces from being able to bring heavy guns forward while the assumption wire on the German front line had been cut proved to be fatal, Lt Gen Keating said in his message.
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.