He was paying tribute to the victims of the February 22 earthquake in New Zealand.
But Japan's current crisis was not far from Prince William's mind, and during his speech he expressed his condolences to the people of the troubled country.
"We honour the memory of those who did not survive," the royal said to a crowd of more than 30,000 people at the national memorial.
"This community... can appreciate the full horror of what is unfolding in Japan."
He said the torment suffered by Christchurch residents gave them an insight into Japan's current situation, where thousands have been killed by Friday's earthquake and tsnuami.
"Our thoughts and prayers are with them too," the 28-year-old told the service, who was mourning the loss of more than 180 people.
The prince is undertaking a five-day tour around disaster areas in Australia and New Zealand.
Photo: © Getty