A machinery operator found the baby's body on Wairau Bar Road, near Spring Creek, on 19 May.
Marlborough area commander inspector Simon Feltham said the post-mortem examination had found the cause of death.
Feltham said police were still appealing for the baby's family to come forward as they held concerns for the mother's welfare.
"We would like to ensure she is safe and can receive any appropriate treatment or support," he said.
Police would also like to discuss options with the family around the baby's remains.
"If this is you, or you've got any information that might help us locate the mother or other family, please get in touch through our 105 service, either over the phone or online at 105.police.govt.nz and quote the file number 260519/9172."
