A woman putting out her rubbish made the shocking discovery of a newborn baby in a bin of an apartment block in Sydney on Monday.

The infant boy was naked and wrapped in a rubbish bag outside the on Harrow Road, Auburn.

NSW Police said they believed the child had been in the waste bin for up to 36 hours, and they are looking for the mother.

One resident of the building spoke of her shock as people who lived there were quizzed by police.

The apartment block’s residents were prevented from leaving as police investigated the discovery.

Sumandra Banthi, 24, lives in the building where the baby was found and said she saw police from her window.

She said residents had been forced to stay inside all day after police established a crime scene outside the building.

“I’m feeling scared,” Banthi said as she headed to the police station to make a statement.

“It’s the first time I have seen [something] like this.”

A general view of Harrow Road in Auburn, Sydney. Photo: AAP

Auburn Police Area Acting Commander Darren Sly said it was too early to determine if the infant had been born alive. Authorities were desperately trying to find the baby’s mother, with action against her not a police priority.

“Our No.1 concern is for that of the mother and the mother seeking urgent medical attention [and] the care I’m sure that she needs,” Sly said on Monday.

“It’s sad and a horrific set of circumstances.”

Sly said it was 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,” he said.

“We’re talking [about] a newborn baby here … you’re going to have those feelings and you’re going to have those memories.”

Banthi said officers spoke to all of the about 20 building residents soon after the discovery, but did not specify exactly what had happened. She said the residents did not really know each other.

Police established a crime scene at the apartment building, which is opposite a school on a suburban street near Auburn’s town centre.

Officers started door-knocking in the area and gathering CCTV footage as part of their investigation.

Anyone who may have seen anything in the area in the past 36 hours that could help the investigation has been urged to contact police.

Lifeline 131 114

beyondblue 1300 224 636

-with AAP