Baby boy found dead inside storm drain was a newborn

There's no evidence to suggest the baby boy was washed through the drain, police say. (Stefan Gosatti/AAP PHOTOS)

A newborn baby boy found dead in a suburban stormwater drain was placed there in recent days before being discovered, police say.

Two workers carrying out routine maintenance made the confronting discovery in Perth's northern suburbs on Monday afternoon.

"The baby down the stormwater drain is, in fact, a little baby boy," Detective Senior Sergeant Stephen Cleal told reporters on Tuesday.

"Evidence suggests it is a newborn child ... (and) that the little boy came to be in that location only in recent days."

There was no evidence to suggest the boy was washed through the drain, Det Sen Sgt Cleal said.

"What we believe is the child was placed in that drain at that location on La Salle Road in Alexander Heights," he said.

Homicide detectives made an urgent plea for the baby's mother to come forward amid concerns for her health.

"We want to help that mother. That is our priority," Det Sen Sgt Cleal said.

Police are pleading for the baby's mother to come forward amid concerns for her health. (Stefan Gosatti/AAP PHOTOS)

Investigators are working with hospitals and medical centres in a bid to find the mother.

They also searched bushland adjacent to La Salle Road and conducted a forensic examination at the scene, including inside the stormwater drain.

"This is a highly distressing and confronting situation for the community ... in that I include the two workmen that come across this shocking and confronting scene," Det Sen Sgt Cleal said.

"We are working with them and my heart goes out to those two."

Workers were clearing the drain of debris and checking for blockages when the "chance discovery" was made.

Premier Roger Cook says the discovery of the baby's body is distressing. (Lukas Coch/AAP PHOTOS)

WA Premier Roger Cook earlier said the baby was believed to have been inside the drain for about a fortnight.

He described it as a "horrifying scenario".

"Those workers that discovered the child's body were confronted with the most distressing of circumstances," he said.

"My heart goes out to everyone involved in this horrible, horrible series of events.

"This goes to the heart of everyone's sense of humanity, a small child whose life has passed under tragic circumstances."

Police have urged people with information about the incident or CCTV recorded in the area to come forward.

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