Three people were on the footpath on Maraekakaho Rd, near the intersection with Francis Hicks Ave, when a car travelling on the wrong side of the road mounted the footpath and struck them, a police spokeswoman said.
All three were hurt, one critically, she said.
Joyce O'Callaghan was inside her nearby Lowe St home when she heard a car "speeding around the corner" on to Maraekakaho Rd, and then a "big bang".
A neighbour later told her one of two men with the baby had saved the infant's life, O'Callaghan said.
As well as the critically injured person, two others were hurt and needed medical treatment, the police spokeswoman said.
They wanted to speak to anyone in the area at the time of the crash.
"If you have any information please contact Police on 06-877 0609 quoting job number P040089521. Alternatively, information can be given anonymously via Crimestoppers on 0800 555 111."
Chavez Farquhar lives on Lowe St and was driving his partner to work about 11am when he was stopped by police near the intersection with Maraekakaho Rd.
"The whole road was shut off and we were told, 'You can't go through'. I saw an ambulance, about four police cars and a group of people gathered on the sidewalk with a couple of police officers."