Jacinda Ardern has another reason to celebrate today, other than formally becoming Prime Minister - her sister Louise has had a baby boy.

After addressing a raucous crowd of supporters at Parliament, Ms Ardern told media that her nephew had also been born today.

The birth was the reason her mother Laurell was not at the ceremony at Government House this morning, where Jacinda Ardern was officially sworn in as Prime Minister.

Laurell Ardern was with her other daughter Louise for the birth in London.

Ross Ardern, Jacinda's father, stayed behind to attend the Government House ceremony.

The new Prime Minister also had some youthful support from her partner Clarke Gayford's two nieces, 3-year-old Rosie and 5-year-old Nina Cowan, who walked with Ardern and Gayford as she entered Parliament for the first time as Prime Minister.