They wrote: “For your friendship, leadership and support over the years, not least at the time of my grandmother’s death. Sending you, Clarke and Neve our best wishes. W&C.”
Clarke Gayford is Mrs Ardern’s fiancé and Neve is their four-year-old daughter.
Mr Hipkins has less than nine months before contesting a tough general election, with opinion polls showing his Labor Party trailing the Conservative opposition.
He has promised a “back to basics” approach.
“This is the greatest privilege and responsibility of my life,” Mr. Hipkins said at the ceremony. “I am energized and excited by the challenges ahead.”