"One of the sad facts for New Zealand is that everyone knows someone who has taken their own life. We're a small country, less than five million people, but last year more than 600 people committed suicide," Ardern said.
It was an issue that was deeply personal for her, having lost friends to suicide, and she said she did not have to look far in her Cabinet to find colleagues in the same position.
"When you provide service within schools that is incredibly beneficial - so we made a decision therefore that we would roll out through every state secondary school in New Zealand, nurses in our schools that work within health teams, knowing that a young person might not always go to their counsellor but they will often seek support around sexual heath or drugs and also just every day issues.
"Just having those nurses available has proven through evidence to be a really successful way of really dealing with youth mental issues.
"We're particularly focused even on primary school-aged children in areas where we unfortunately have had the occurrence of severe earthquakes, because we know that's affected our children and young people's resilience."
She highlighted the Government's work in its first 100 days in office to commission a review of New Zealand's mental health and addiction services, and brought with her a copy of the report which was produced late last year.
The Government will respond to it in March, but Ardern told the audience mental heath was one of five priorities in this year's so-called wellbeing Budget, touted as the first of its kind in the world.
"More than that, on an ongoing basis, we need to measure our success as a nation beyond just economic measures of measures of wellbeing."
Prince William told of the impact his work in the Air Ambulance Service had had on his own mental health, and one job in particular that hit him hard as a father. He did not go into detail about the incident but said he sought help to deal with it. Had he not, he said, his mental health issues might have been worse later.
"I know that if I hadn't taken the action I that did then I would have definitely gone down a slippery slope and I would have been dealing with mental health on a different level."
William is known for his advocacy on mental health issues. He, his wife Catherine and brother Prince Harry founded the mental health charity Heads Together.
Ardern agreed that while mental heath was discussed in New Zealand society, it did not reduce the stigma attached to mental health issues.
She paid tribute to former All Black Sir John Kirwan for his public advocacy on mental health.
"That, I think, in some ways, was quite ground-breaking. It was the start of some of those conversations, not just at a national level but personalising it."
She also praised William. "I just don't think you can underestimate the power at an international level of you having spoken so openly.
"Policies matter, programmes matter, but actually conversations matter enormously."
