Teenage parents abandoned their newborn baby at Middlemore Hospital, prompting fears they were trying to hide the birth from their own parents.
A spokeswoman said the young couple had just "packed up and left".
She said the baby was in the care of Child, Youth and Family but she could not comment further or indicate if it was a boy or a girl.
A source told the Herald the mother was a 14-year-old who had tried to hide her pregnancy from her family.
Authorities would not confirm the girl's age for fear of identifying her but did say she was a teenager.
CYF northern director Marion Heeney said it was unfair to call the case an abandonment. "There was initially some concern that the baby might have been abandoned. While the baby was left there [on Monday], the family had made it clear to hospital staff they wanted the baby adopted."
Ms Heeney said CYF would work with the family in coming weeks as the adoption process got under way.
She said there were many reasons mothers felt they were unable to care for a child and that adoption was in the best interests of their child.
In 2007-08, 259 New Zealand children were adopted out - 77 to non-relatives.