Mr Sia ran to get the attention of staff, and then rushed back to the car to try and calm the child.
"People were running through the store, saying 'who left a baby in a car' but they couldn't find anyone," he said. "People wanted to smash the window and call the police."
Staff were extremely worried he said, and there was an argument about the best thing to do. Just as the group were about to break into the car, Mr Sia found the mother, who was completing her shopping at the checkout.
"I told her, you can't leave a baby in the car."
The woman was very apologetic, and said the baby hadn't wanted to go into the shop with her. He said she was new to New Zealand and didn't know the law.
Mr Sia later posted on Facebook about the incident to warn others of the dangers of leaving children in cars.
"I'm new to New Zealand too so I thought it was part of my obligation to let people know."
Police said they hadn't received a report about the incident.
Last year, a 16-month-old child died from after being left in a car by his mother.
A Wanganui woman was charged with manslaughter but discharged without conviction.
The boy died of heatstroke and dehydration. He was discovered only after his creche texted and then phoned to inquire where he was.
The law states that young children must never be left alone in a house or vehicle - they need constant supervision.
It is illegal to leave a child under the age of 14 years without reasonable provision for their care.
