Holding the mum's keys, the bystander points her finger in her face, telling her she's "waiting for the police to arrive".
Western Australia Police arrived a short time later, issuing the mum a warning.
The local Domino's told 7 News the mum was finishing work and just had the child with her for her final delivery.
Her husband, and the baby's father, also works at the same pizza shop and had arrived at the business to start work, handing over the couple's car and child to his wife.
The woman who took the keys and called the police insisted her first concern was for the baby.
Despite some people lashing the mum online, others lamented she was likely in a difficult situation.
"It's just horrific that has to happen, that people are forced to do that to make a living," Transport Workers Union WA spokesman Tim Dawson said.
"It is slave labour."
Domino's delivery drivers are paid under the fast food industry award however, other delivery drivers working for companies like Deliveroo, Uber Eats and MenuLog can be paid as little as $5.90 an hour.
