After several people lifted the heavy slab, the baby boy - wrapped in a striped hospital blanket with his umbilical cord cut and clamped - was found at the bottom.
"It was disturbing (to see) how the child was placed in the drain," Quakers Hill police Inspector David Lagats said.
"We all thought the worst but the baby was still alive."
Inspector Lagats said there were no signs of physical injury but tests will be carried out to see if he suffered internal injuries.
Police doubt the baby would have survived as temperatures in Sydney's north west reached almost 40 degrees on Sunday.
"It was already undernourished, and dehydration would have taken affect so we would have had grave fears for the child's welfare had it been exposed to this weather for the rest of the day," Insp Lagats said.
The baby remains in a serious but stable condition at Westmead Children's Hospital.
His 30-year-old mother was questioned at Blacktown Police Station but was taken to hospital on Sunday night.
It's expected she will face police again when she is discharged.
The baby is now in the care of the NSW Family and Community Services (FACS) Minister.
It will be up to the state agency to assess how - or if - the mother and baby can be reunited.
FACS western Sydney director Lisa Charet said often with abandonments, people were in a place of depression.
"There's been cases where mothers have had post-natal depression, or those sorts of issues, or they're very young and they don't know where to go to for help," she said.
Labor Senator Helen Polley has called for emergency hatches in places such as fire stations, police stations and hospitals where people could leave a baby "without fear of criminal prosecution."
- AAP
