With a mock security pass that lists her as the "First Baby" of New Zealand, three-month-old Neve Te Aroha made her United Nations debut when her mother, Prime Minister Jacinda Ardern, spoke at a peace summit in the General Assembly.

Ms Ardern planted a kiss on baby Neve during the inaugural high-level Nelson Mandela Peace summit, a day before leaders address the UN General Assembly.

Her partner Clarke Gayford, who is the baby's full-time caregiver, sat with the New Zealand delegation and held Neve as Ms Ardern spoke.

He also tweeted a photo of the security pass and joked that Neve had kept them awake, without any regard to different time zones.

"Because everyone on Twitter's been asking to see Neve's UN ID, staff here whipped one up," he wrote.

The UN security pass for New Zealand First Baby Neve Te Aroha. ( Twitter: Clarke Gayford )

"I wish I could have captured the startled look on a Japanese delegation inside the UN yesterday who walked into a meeting room in the middle of a nappy change. Great yarn for her 21st."

Ms Ardern, 38, is only the second elected leader to give birth while in office, after Pakistan's Benazir Bhutto in 1990.

She is her country's youngest premier and the first to take maternity leave while in office.

The United Nations was delighted to see baby Neve in the General Assembly hall, UN spokesman Stephane Dujarric said.

Jacinda Ardern kisses her baby before speaking at the Nelson Mandela Peace Summit. ( Reuters: Carlo Allegri )

Reuters