Police say they have not located the mother who left her newborn baby in a car while she went shopping because the person who discovered the infant never reported it to authorities.
The sleeping child was left in a car outside the Porirua Pak'n Save with a note to call a mobile number if there were any issues.
Strangers who found the child waited with it until its mother returned and a picture of the sleeping baby made news around the world last month.
Senior Sergeant Steve Sargent said officers had tried to find the mother but without information from those who saw the child it was almost impossible.
"Unless the person who actually rang the [mother] ... contacts us and gives us the phone number they rang, we really don't have any [leads]," Mr Sargent said.