Ms Ardern is leading a cross-party delegation of MPs with business, religious and community leaders on a short and sharp two-day visit at the invitation of Samoan prime minister Fiame Naomi Mata'afa.
The occasion is Samoa's 60th anniversary of independence, a Pacific first, and the signing of a Treaty of Friendship with New Zealand, which endures to this day.
"We don't have a treaty of this kind with any other country," Ms Ardern said.
"Our unique friendship will continue to expand and deepen.
"And we pledge to recommit ourselves to walk together whatever paths the future may hold for us both as friends and neighbours in the region we each call home."
Samoa has been a diplomatic go-to nation in recent months, playing a key role in the Pacific Islands Forum, and signing bilateral deals with China.
The occasion was a cultural counter-balance to assertive displays by greater powers, including China and the USA, in the Pacific.
On display on Monday was a deeper diplomacy, reinforced by people-to-people ties.
An estimated 250,000 Samoans live in New Zealand compared to roughly 200,000 in Samoa itself.
In New Zealand, the anniversary has brought reflection on its half-century of colonial rule.
In 2002 - on the occasion of the 40th anniversary of independence - then NZ prime minister Helen Clark travelled to Samoa "troubled by some unfinished business", issuing a formal apology for past injustices.
Chief among them was a decision in 1918 to allow a flu-ridden ship to dock in Apia, producing an influenza epidemic where one in every five Samoans died, and the killing of non-violent protesters.
New Zealand's Pacific Peoples Minister Aupito William Sio told AAP these events were still very present in the collective Samoan memory.
"There were things done to chiefs and others - banishment ... imprisonment, the removal or the stripping of titles. All of those things may have occurred long ago but they've been kept in songs and stories and oratory that are passed on," he said.
"So we're working through all of that."
Speaking on the homestead of writer Robert Louis Stevenson, where the treaty was signed, both prime ministers paid respect to grievances but preferred to look forward.
"(Stevenson) wrote some memorable phrases during his short lifetime, including that 'a friend is a gift you give yourself'," Ms Ardern said.
"I have no doubt that those who came before us had similar sentiments when they gifted us the Treaty of Friendship 60 years ago."
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