The mother of a newborn baby boy left inside a drain near Sydney's M7 motorway has been found.

The baby's cries alerted cyclists who were riding alongside the motorway at Quaker's Hill in Sydney's west on Sunday morning.

He was rescued and police said it appeared the baby had been abandoned and dropped 2.5 metres through an opening in the drain.

Inspector David Lagats said officers were horrified by the discovery.

"It was already undernourished and dehydration would have taken effect," Inspector Lagats said.

Police doubt the child would have survived in Sunday's heat if he had not been found so early in the morning.

They believed the baby was born in hospital and after checking hospital records and doorknocking the area, they identified the baby's mother as a 30-year-old Quakers Hill woman.

Police said the woman was taken to Blacktown Hospital after complaining of an illness during questioning.

The boy remains in a serious but stable condition at the Children's Hospital at Westmead.

Earlier, Family and Community Services district director for Western Sydney Lisa Charet said the parents must have been feeling desperate.

"Often when this sort of thing happens, people are in a place of desperation," she said.

"There has been cases where mothers have had postnatal depression or those issues or are very young and don't know where to go to for help. That's where we come in because we can provide help.

"We are really worried about primarily his welfare and mum's."