Police investigating the death of a newborn baby boy at a house in west Auckland are refusing to say how the baby died.

A post-mortem examination has been carried out on the body of the baby which police spokesman Kevin Loughlin said was born premature.

Mr Loughlin said the post-mortem revealed the baby's gender.

"The cause of death is not being disclosed," Mr Loughlin said.

Detective Sergeant Sean Hyland-Mills said police went to an address in Barron Drive, Green Bay, yesterday afternoon and found the body.

"Medical professionals and relatives are being spoken to and a scene examination is in progress.

"The address remains in police control while our investigations continue," Mr Hyland-Mills said.

Police are still at the house, which is cordoned off, and are not revealing details of the investigation.

TV3 is reporting that a neighbour said the baby was dropped over a fence.

Police have erected a tarpaulin between a garage and a fence.

Barron St resident John Morgan said the house was opposite a playground and near a primary school.

"The kids sitting at the park are looking across at a house that's all taped off surrounded by cops, it's not very nice... It's a nice area, quiet area, there's lots of kids around here and the park's always full of kids."

His two eldest children, aged 14 and eight saw the action while walking home from school.