An investigation is under way after reports of a woman allegedly assaulting a baby in a carpark in Kumeu, west Auckland, on Sunday.

A Facebook post on the local Facebook group raised the alarm as a witness posted a photo of the accused and a description of alleged events.

The post, which has since been deleted, said the woman in the photo had "violently physically abused a baby in its car seat", "ramming it its chest over and over so hard its whole body was jolting".

The Facebook user added that the baby was "screaming crying" and the woman was "screaming at it to shut the f... up and stay the f... there".

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The witness says the child in question was not the woman's child and added that she called police after the event.

"I am shaking and feel physically sick," she added.

Police have confirmed an incident occurred in the carpark of the New World supermarket in Kumeu on Sunday afternoon and they were looking for anyone who might have witnessed the events.

"As the matter is currently under investigation police are unable to comment further. However, we would like to hear from anyone that witnessed the incident which occurred at approximately 4.30pm," Detective Sergeant Jason Cleeton told the Herald.

• If you have any information that could help the investigation, contact Detective Sergeant Jason Cleeton from Henderson Police on (09) 213 9736. Information can also be provided anonymously to Crimestoppers on 0800 555 111.