A New Zealander mum has escaped facing potential charges after leaving her newborn baby in her locked car in a car park outside a New Zealand supermarket so she can do some shopping.

Although bystanders took pity and eventually called to the mother to attend to her child, Justin Rakena, a Senior Sergeant, said no one among them made a formal complaint against the mother.

"(Incidences like this) need to be taken on their merits and often it's a mum that's run into a shop, for example, and is only away for five minutes," Mr Rakena was quoted by the New Zealand Herald.

"Absolutely (it should be reported to police), but it doesn't mean to say we'd prosecute. I would suggest the majority of people in that situation aren't prosecuted."

Photo of a sleeping newborn baby straddled to a car baby seat heavily circulated over the weekend in New Zealand. On top of the blanket covering the baby was a handwritten note that said, "My mum's in doing the shopping, call her if I need anything." A cellphone number was indicated.

One family said they were speechless when he said the sleeping baby inside the parked car at Pak 'n Save supermarket on Saturday at 9am.

Upon seeing the baby, the father of the family said they waited by the car "for a little bit, wondering if the mum was just going to be two seconds and come back."

"Eventually my wife said she's not going in the supermarket without someone being there with the baby."

The mother eventually got called out to attend to her child when another two passers-by arrived at the carpark.

"Babies can dehydrate quickly and become very distressed. So for a newborn that's one of the key issues. Plus there's a security issue - a small baby is unable to defend itself if need be, or call for help, or anything like that," Sue Campbell, Plunket national child safety adviser, was quoted by the New Zealand Herald.

The identity of the baby and the mum from Porirua remained unidentified.