The newborn's body was found in a stream on Burnside Road in Ardmore, on the southern outskirts of metropolitan Auckland, on Saturday night.
Police subsequently cordoned off staff quarters at the nearby property of top harness racing trainer-driver Tony Herlihy.
Detective Senior Sergeant Darrell Harpur said police had spoken to the baby's mother and found she was born at that property last Wednesday.
Mr Harpur said the circumstances surrounding the baby's death were still not clear.
"A post-mortem examination was carried out this morning which confirmed that the body was that of a baby girl of at least 35 weeks gestation," Mr Harpur said.
"We await the results of pathology tests and ongoing inquiries.
"We have interviewed the mother and no charges have been laid at this stage."
Anyone who may have seen something suspicious while driving along Burnside Road between 4.30pm and 7pm on Wednesday is asked to call police.
