The woman, who spoke on condition she was not identified, said she was "quite shocked".
The offender quickly fled.
The woman reported the attack to police and New World staff.
She said she had been asked why she did not get a better look at the thief but her priority was her baby.
"My son was screaming his lungs out," she said.
The baby was later checked over and found to be unharmed by the fall.
The woman said losing her wallet with everything in it wasn't major compared to her baby's health, but she wanted people to know there were thieves out there.
"I wasn't focusing on anything else but trying to put my groceries and my son in the car when they came," she said.
"I was told while I was there that people had been targeting elderly and disabled people and mothers with babies."
The woman said she felt bad for the New World staff because it's not their fault and the supermarket had been especially busy.
Brookfield New World owner and operator Dean Waddell did not return phone calls.
Senior Sergeant Glenn Saunders confirmed police were investigating the incident. He said he was not aware of any trend involving people being targeted in the area by thieves, but warned that people needed to take precautions to stay safe.
"As police we always encourage people to be aware of their security and safety in the environment they are in. For example, a woman carrying a purse and pushing a pram, she probably has several different things on her mind. Maybe her purse would be safer in with the child.
"There are people out there who prey on the vulnerable," Mr Saunders said.
"The more secure your items, the less likely of a victim you will be."
