Prime Minister Jacinda Ardern said she was "humbled" to be asked by Prince William to speak on his behalf at a climate change charity this morning, New Zealand time.
She revealed that William had personally asked her to deliver the speech after he cancelled his trip to New York following the death of his grandmother, the Queen.
"I am an exceptionally poor substitute," Ardern said.
William is a co-founder, along with Sir David Attenborough, of the Earthshot Prize - an environmental charity that finds and funds innovative solutions to environmental problems.
The title name "Earthshot" is a play on President John F. Kennedy's "Moonshot" - a challenge to his country to put a man on the moon within a decade. Kennedy's daughter Caroline Kennedy attended the event.
Ardern was introduced by William's partner in the Earthshot Prize, billionaire philanthropist and former New York Mayor Michael Bloomberg.
He described Ardern as a "global leader on climate change".
In a pre-recorded video speech, William thanked "Jacinda and Mike" for appearing at the event.
Ardern compared the fortitude needed to fight the climate crisis with the life of the late Queen.
"She is someone who was focused, committed and demonstrated what be achieved when you show fortitude and longevity.
"We need all of those same characteristics to take on the environmental challenges ahead of us," she said.
Ardern said that looking after the environment in this country came from a Māori worldview which saw that "people and nature are kin and family".
Ardern was one of a number of speakers at the star-studded event, held in the gilded rooms of New York's Plaza Hotel, once owned by Donald Trump.
Guests included Queen Rania of Jordan, Mayor of Paris Anne Hidalgo, actor Matt Damon, and Oscar the Grouch, of Sesame Street fame.
Damon joked that there was no "planet B", which people would know if they watched his 2015 film The Martian.
"I spent half the movie trying to grow one potato," Damon joked.
The event also featured the founders of environmental businesses and charities involved in the likes of rewinding and renewable energy generation.
Ardern talked up New Zealand's own climate efforts, including the Zero Carbon Act, which legislated New Zealand's target to keep global warming to 1.5 degrees above pre-industrial levels, banning non-recyclable single-use plastics.
Ardern also said New Zealand was "committed to becoming the first country in the world to price agricultural emissions".
The Government's current plan is to have a pricing mechanism for those emissions, but agriculture will not pay the full price - getting what is an effective subsidy.
Ardern will spend the afternoon in more familiar company, appearing on a panel with her old boss, former Prime Minister Helen Clark.
Clark led the independent panel for pandemic preparedness and response and will be interviewing Ardern about leadership during a pandemic.
Ardern will appear alongside Ellen Johnson Sirleaf, who was president of Liberia during the Ebola crisis.
Elsewhere at the United Nations today, United States President Joe Biden responded to a speech by Russian President Vladimir Putin.
Biden laid the blame for the Ukrainian invasion squarely at Putin's feet and accused him making "irresponsible nuclear threats" in relation to the war.