Fears that COVID-19 might hurt attendance didn't come to pass. Thousands of Kiwis braved the 5am start, journeying to the grounds where the Treaty of Waitangi was signed by Maori chiefs and the British Crown 181 years earlier.
Beginning in darkness, those gathered heard prayer and joined together in song, singing waiata and the national anthem.
Ms Ardern called New Zealanders "hardy, practical, creative people, working towards a common purpose".
"Today we mark the Treaty of Waitangi, Te Tiriti o Waitangi," she said.
"We acknowledge our partnership, constantly learning, striving for better, holding hope for the future."
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Since becoming prime minister in 2017, Ms Ardern has made a tradition of government members cooking a free breakfast for all attendees to the service.
"When I first became prime minister I was running through the usual program that every other leader has had here and I was told that there's usually a prime minister's breakfast and it's hosted at the Copthorne (Hotel)," Ms Ardern said.
"My view was that that wasn't as open and didn't include as many people as I would like."
New Zealand has commemorated Waitangi Day as a public holiday since 1974, for two years as "New Zealand Day", before reverting to the name of the foundational treaty.
"Every Waitangi Day is a chance to reflect on ... how are we progressing, our partnership, our relationship, but also constantly looking forward with hope," Ms Ardern said earlier this week.
"There are things that we need to do better. But I do absolutely believe that you get a sense here of real optimism about what can be achieved together."
Ms Ardern's optimism has been met with a healthy dose of cynicism and on-the-ground truths by some Maori leaders.
On Thursday, political leaders heard stories of social disengagement, police harassment and entrenched Maori poverty.
Hirini Tau, a local Maori elder, told the PM: "don't just come sit down once a year and go back to Wellington".
Just as in Australia, Indigenous Kiwis are overrepresented in prison populations, have worse health outcomes, and underrepresented in positions of power.
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As is now traditional, Ms Ardern has spent several days on the treaty grounds in the lead-up to Waitangi Day.
At those gatherings, she has trumpeted her government's achievements for Maori. That includes the teaching of local history in Kiwi schools, a new public holiday for Maori New Year, and growth in Maori-specific agencies.
Her government also has five Maori ministers and 15 Maori MPs in her caucus of 65, both records.
But issues such as reducing inequality, getting Maori off the housing waiting list and into homes, and increasing Maori employment remain.
Ms Ardern says making progress on those issues is "absolutely why we are here as a Labour Party".
"We will always be the first to put our hand up and say we've got more to do," she said.