The four-day event in Chicago is celebrating the official nomination of Vice-President Kamala Harris as presidential candidate in the November election.
President Joe Biden was the keynote speaker on the first night, capping a handover to Harris after he was pressured to quit the race last month by party leaders worried the 81-year-old incumbent was too old to win or serve another four years.
Harris, 59, made a surprise appearance on the first night, drawing cheers from the crowd as she vowed to defeat 78-year-old Republican rival and former President Donald Trump.
Ardern will be in the arena when former US President Barack Obama and former First Lady Michelle Obama make their keynote speeches on the second evening.
"It is a real privilege to be able to witness this moment in time," Ardern said.
"You could see from the room a number of international guests and I think in part that's because of how important this election is, but also because it feels like a historic moment."
Ardern told a meeting of Democrats on the sidelines of the convention that people were seeking a new kind of politics.
"If there's any lesson I learned role in politics, it was that ultimately the politics of any country is for that nation to decide. And for that people to decide.
"But I certainly get asked a lot generally about politics at the moment - the global state of politics.
"I do think if you look around the world there is a call from voters to see that positivity from politicians to try and reduce that sense of division.
"And so that's something I have an interest in now in my life post-politics. That's one of the reasons I started the Field Fellowship."
The fellowship is a programme for political leaders about leading with kindness and empathy.
Ardern was also participating in a panel event to discuss policy ideas alongside other current and former international leaders.
- RNZ / Reuters
