Hundreds of people have been forced to evacuate their homes as a storm battered the country this week.
On Friday, the duke said on Twitter: "Seeing the devastation caused by the floods in New Zealand is a reminder of the herculean effort we must all make to protect our planet.
"I know you will come together, as you have done before, to support those in need. Catherine and I are thinking about all those affected."
The statement comes after around 230 homes in the town of Nelson, near the northern tip of South Island, were evacuated on Wednesday when the Maitai River flooded.
The storm also caused a home to slip down into a gully in the town of Tahunanui in the area.
The military were called in to patrol the area overnight.
Another 160 homes in Westport, on South Island's West Coast, were also temporarily evacuated.
Elsewhere, residents in the northern part of North Island became cut off after landslides, fallen trees and floodwaters blocked access to roads.
Some schools and businesses were forced to close, while airlines cancelled some flights.