Emergency services found the body of a newborn boy in Auburn, in western Sydney, after responding to reports about 11.20am on Monday.
The baby was found naked inside a rubbish bag in an apartment block bin on Harrow Road by a local resident.
Auburn Police Area Acting Commander Darren Sly said it was believed the baby had been in the bin for up to 36 hours but it was too early to determine if he had been born alive.
Authorities were desperately trying to find the baby's mother, with action against her not a police priority.
"Our number one concern is for that of the mother, and the mother seeking urgent medical attention (and) the care I'm sure that she needs," Acting Commander Sly said.
"It's sad and a horrific set of circumstances."
He said it was very traumatic for the woman who had found the baby while disposing of rubbish and for emergency service personnel.
"My officers are professional, they'll do their jobs, but like anybody, they're human," Acting Commander Sly said.
"We're talking (about) a newborn baby here ... you're going to have those feelings and you're going to have those memories."
Police established a crime scene at the apartment building, which is opposite a school on a suburban street near Auburn's town centre.
Officers have started door-knocking in the area and gathering CCTV footage as part of their investigation.
Anyone who may have seen anything in the Harrow Road area in the past 36 hours that could help the investigation has been urged to contact police.
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