"My mum stayed by the car and I went back to the shop and yelled out: 'Who's left their child unattended in the car?'. I said: 'If you don't get out there now, I am going to break your window'," Williamson said.
"I went back to the car and this lady came out about three minutes later, because she had paid for her things. I said: 'I am not trying to be horrible to you but this is unacceptable, you can't leave your child in the car'."
She said the woman said her baby was "fine", put her toddler in the car and drove off.
"It is just really upsetting. It must have been 28C out there yesterday and in 20 minutes, your car can rise almost 20C in the internal temperature sitting in the sun. That's organ failure, brain damage type of material there."
Williamson rang the police immediately afterwards and gave them the car registration number.
A police spokeswoman said they were informed of the incident about 2.30pm on Tuesday.
"Police will be speaking with those involved."
On Sunday, a similar incident occurred when a woman left two young children in a parked car while she went shopping on a 33C day.
Chrystal Alatasi came across the children in the car parked in the New World supermarket at Greenmeadows in Napier. However, the child was old enough to open the door.
Alatasi said the heat blast was like opening an oven door when the older child was coaxed to unlock the car door.
Police issued the woman with a warning and reported her to Oranga Tamariki.
