The sanctuary will prevent fishing and mineral exploitation in an area where marine scientists regularly discover new marine species.
Environmental groups applauded the move, saying it added to a network of protected areas in the Pacific that now covered more than 3.5 million square kilometres.
"We congratulate the government for taking decisive action to protect this incredibly special area from mining and fishing," WWF New Zealand chief executive Chris Howe said.
"This decision puts New Zealand back at the forefront of marine protection on the global stage."afp