Police cordoned off an area around the playground at Myers Park following the assault on Sunday afternoon. Hato Hone St John sent two ambulances, a rapid response vehicle, and an operations manager to the scene.
Inspector Jim Wilson said the man was not known to the victim and that both the victim and the offender required medical treatment. "This was an isolated incident, and there is no ongoing risk to the public," Wilson said.
Police confirmed the 20-year-old man was taken into custody in connection with the assault. The child remains at Starship Children's Hospital with critical injuries.
The incident occurred in a busy part of central Auckland, and police have not indicated any further concerns for public safety.