She looked around and saw it was coming from a silver station wagon parked in the designated caregivers carpark.
"It was screaming and that's what made me look over. I put my face up against all the windows in the car and was like holy crap there's no one in this car and the baby is by itself".
The woman, who had her own child in tow, dashed back into the supermarket and asked management to help.
"I said 'OMG can a manager come out here - there is a baby in a car'. They acted really really well and did everything they could instantly."
The woman said the supervisor put an announcement over the loud speaker while she went back outside with other staff members.
"We tried doors and nothing would work. And then I was banging on the windows to distract the baby so it wouldn't get so hot."
She said the baby was screaming the entire time and a group of about eight people - including staff - had gathered around.
About 10 minutes later, the mother emerged with bags of shopping. The woman did not know how long they baby had been left for before she heard it.
"She just walked out and saw that everyone was around her. I think she got quite embarrassed and I just had my hand over my mouth because I was in absolute shock and she unlocked the car and opened up the back door and pulled the cover off the baby.
"When she got back to the car and opened the door and the baby's face was red and round the collar of the baby's shirt was all wet - not of sweat I don't think but of tears. Of the baby being distressed.
"The baby started to calm down and the mother touched its tummy," the shopper said.
"She was like 'he's fine, he's fine - I've been in hospital sorry' and closed the door and took off."
The woman said she was left standing there in shock and posted the photo to try and get help for her.
"And that's why I wrote the post because it's dangerous and could kill her baby.
"Even if it was just 10 minutes in the supermarket - the plastic cover is a little bit dangerous.
"I just feel sorry for the lady - she obviously needs help."
The advice from advocacy group Safekids Aotearoa was to never leave a child in the car even for a minute.
Countdown spokeswoman Kate Porter said the supermarket chain was aware of what happened and was pleased it was resolved quickly.