The prime minister celebrated the launch of her long-promised families' package with a message from her couch - baby Neve in arms.

Going live on her Facebook page, Jacinda Ardern made her first public comments since she and partner Clarke Gayford left Auckland Hospital following the birth of their first child, Neve Te Aroha Ardern Gayford.

The $5.5 billion package is expected to boost the incomes of an estimated 385,000 families by $75 a week by the time it's fully implemented, in 2020-21. About 650,000 families are expected to gain an average of $14 per week by that time.

JACINDA ARDERN/FACEBOOK PM Jacinda Ardern talks, baby in arms, about the new families package.

Looking happy and calm, with baby Neve bundled in a blanket on her lap, Ardern spoke of the package's inception - particularly the "First Start" payment for newborns.

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"Many years ago now, one element of it at least - the Best Start payment - I started designing, on the floor of a friend's house in Hastings," she said. Ardern would have been the Opposition spokeswoman for children at the time.

"And it was based on all of the research and evidence that was telling us the most important period of a child's life, is those first early years.

"And equally, at that time, that was when children in New Zealand were experiencing the most persistent poverty," Ardern explained.

The Best Start tax credit offers the families up to $60 a week for a child's early years - universal in the first year, but means tested in the following two years.

STUFF Prime Minister Jacinda Arden and partner Clarke Gayford presented their new baby daughter at Auckland Ciy Hospital after her birth.

Ardern said it was the part of the package she was most proud of. Because it came into force on July 1, the Prime Minister was not eligible herself, but any family with a baby born or due from July 1 would receive the payment.

Talking to her phone camera, Ardern also explained the other details of the Families Package, which included the Winter Energy Payment, a boost to working for families and some additional measures aimed at low income families.

She also addressed the fuel tax - a double hit for Aucklanders - that came into force today, and made the point the families package gains, "far outstripped" the fuel tax on a weekly basis.

While only the top of baby Neve's head was visible, Ardern did share some insights to her life as a new mum.

With her new baby in her lap she revealed "this is my permanent state at the moment".

"Either breastfeeding or this," she laughed.

Ardern also promised "she wasn't too tired" and was "doing great".

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