It was a shock to see the hand-written note resting on top of the baby's blanket, he said.
"It was written from the baby's perspective, and it said, 'My mum's in doing the shopping, call her if I need anything', and it had the cellphone number.
"We waited there for a little bit, wondering if the mum was just going to be two seconds and come back. And my wife said, 'I'm not going in without someone being here with the baby'."
Eventually another two passers-by waited by the car, with one of them phoning the mother and telling her to come outside.
"We had gone in [to the Pak'nSave] before the mother had come out because we had a baby ourselves that was in a hurry."
The man said the baby looked well-cared for.
"As parents ourselves we know it is hard to get a baby to sleep, and once you start moving them they can wake up ... we thought it was just a silly decision by a tired mother."
Plunket national child safety adviser Sue Campbell said the risks of leaving a baby or young child alone in a car were significant.
"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."
Ms Campbell said the circumstances of the mother were not clear, and caring for a baby could be overwhelming, especially for those without ready family support.
"There are numerous support groups out there for parents ... if they are in a place where there is a Plunket Family Centre, there's the opportunity to take your baby in and get some support, and perhaps a few hours sleep while somebody's watching your baby for you."
All parents would know how trying looking after a baby could be, she said. "If a mother is having difficulty coping, there is concern for the mother as well."
A police spokesman said it was illegal for a parent or guardian to leave a child under the age of 14 alone for an unreasonable time or in unreasonable conditions. Doing so could result in a fine of up to $2000.
"People have been prosecuted for leaving children in cars in the past. But equally, short of prosecution, they may well be referred to Child, Youth and Family."
Last year Child, Youth and Family began an investigation after a baby was left distressed and alone in a car in Huntly.
And police were called after five children, ranging in age from five months to eight years, were left in a van unattended in Auckland's SkyCity carpark last year.
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