Cleaners at the Ministry for Vulnerable Children found a girl, alone, inside the government office in Auckland after hours.
The ministry's deputy chief executive Glynis Sandland confirmed to NZME a teenage girl had been left behind after workers left for the day.
She said the situation, which happened six weeks ago, was "completely unacceptable".
"We would not want any child to be left alone like that, or for us to not know where a child is."
Sandland said police had brought a 14-year-old girl into the offices about 1pm, and told a supervisor.
The girl was placed in the quiet room for children, in the Auckland offices and went to sleep.
"A family member later arrived to collect her, but left before staff went and got the girl."
Sandland said subsequently staff left for the day, just before 7.30pm and the girl was left on her own for about half an hour before the cleaners discovered her just before 8pm.
The deputy chief executive said an employment investigation was under way, but was unable to elaborate further on what shape this would take.
Earlier reports showed an Auckland social worker told Radio New Zealand a boy was left behind, forgotten and alone, when staff left the Takapuna office one night recently.
However, it appears she was mistaken about the child's gender.
The social worker told the radio station it was unusual to forget a child, but that children were frequently waiting all day while social workers looked for a place for them to stay.
"It wouldn't be unusual for children or young people to be waiting in the office all day because there's no placement for them or a placement has been arranged but it's not ready."
The social worker told RNZ it could be difficult to find a placement.
"You've been looking all day, you've been ringing all day, you've been negotiating with a regional office."
She also said the rebranding of Child Youth and Family to the Ministry for Vulnerable Children/Oranga Tamariki in April had changed little.
Social workers were still typically looking after 20 to 30 families, instead of the recommended 15, she said.
The ministry has been approached for comment.
Sharon Chandra is a senior associate at Turner Hopkins where her principal area of practice includes family law matters.
She told NZME that in her years of practice she had not encountered a child being forgotten in an office.
"In my experience I would think this kind of thing is rare."