"A youth was arrested, and has since been referred to Youth Aid in relation to this incident."
The witness didn't see the impact, but overheard police speaking to a man he believed was the toddler's father.
"He was absolutely fuming. As far as he was concerned they'd tried to kill his little girl ... the woman [with him] was in tears but the baby was okay."
The man who contacted the Herald estimated the toddler was aged about 2.
Four teenage boys were at the scene and six police cars arrived after emergency services were alerted, the man said.
The windscreen of the family's car, a white station wagon travelling westbound, was smashed on the passenger's side.
The incident was "bloody horrible" and he kept thinking about the death of a man killed when an 8kg block of concrete was deliberately dropped from the Princes St overbridge on Auckland's Southern Motorway.
Taupō 20-year-old Christopher Currie, a King Country under-20 rugby team prop travelling to a game, died instantly when the block went through the windscreen of his car in 2005.
Three passengers were hurt when the car subsequently crashed and a 14-year-old boy, Ngatai Rewiti, was later found guilty of manslaughter over Currie's death, and sentenced to four years' jail.