Gayford posted an adorable picture with the tweet of Neve in UN pyjamas.
Gayford also tweeted yesterday about Neve's time at the UN, saying staff had even whipped up an ID card for her.
"I wish I could have captured the startled look on a Japanese delegation inside UN yesterday who walked into a meeting room in the middle of a nappy change."
He suggested it was a good story for her 21st.
Neve had her first visit to the UN General Assembly hall yesterday, watching on while her mother spoke there for the first time to pay tribute to Nelson Mandela.
Ardern was speaking at the United Nations General Assembly hall for the first time for the Nelson Mandela Peace Summit.
She spoke of what Mandela meant to New Zealand - and her father missing Ardern's first birthday because he was a policeman on duty for the 1981 Springbok protests.
As she spoke, Neve watched on, held by her father Clarke Gayford. After speaking, Ardern herself took over, holding Neve as other leaders spoke.
The photos quickly spread in the media and on social media. Samantha Power, a former US Ambassador to the UN under former President Barack Obama, tweeted them, saying: "I cannot stress how much the @UN - and the governments that comprise it - need this."
