A newborn baby survived for six days at the bottom of a stormwater drain in Sydney before being discovered on Sunday.

Police have charged the baby's mother with attempted murder and say it is unclear how the baby survived for so long, apparently without food or water, after he was dumped in the drain on Tuesday last week.

Police will allege the baby, believed to have been born last Monday, was pushed into the drain on Tuesday.

The child was taken to The Children's Hospital at Westmead in a serious but stable condition.

Police said on Monday the baby was in a stable condition.

The discovery sparked a police search for the boy's mother – identified later as a 30-year-old Quakers Hill woman – and calls for the public to withhold judgment until it becomes clear what led to the baby being abandoned.

By early Sunday afternoon, checks of recent hospital births and doorknocking of the area had led police to the mother, who had given birth at Blacktown Hospital.

The woman was arrested after attending Blacktown police station about 2pm on Sunday.

She was refused bail and is due to appear at Blacktown Local Court on Monday.

"Just unbelievable"

Rydalmere resident David Otte said it was "just unbelievable" to find the boy down the 2.5-metre drain along a Quakers Hill bike path next to the M7.

Mr Otte said he had been cycling with his daughter Hayley when, about 7.30am, they were flagged down by two men who had heard an unusual noise coming from a drain about 150 metres from Quakers Road.

"It was so intense. You couldn't not tell it was a baby," Mr Otte said. "We couldn't see it but we could hear it. It was distressed."

It took about seven people to lift the concrete lid off the drain so police could retrieve the boy, who was described by police as malnourished and dehydrated.

The baby's umbilical cord had been cut and clamped. He was wrapped in what appeared to be a striped hospital-issue blanket.

"We were going to get that lid off no matter what it took," Mr Otte said.

"Physically, no one could have fitted themselves down into that drain. A child maybe but not an adult, no way in the world."

"You go through life seeing things but you never, ever imagine you'll see something like this," Mr Otte said.

He said the baby had plastic around his body.

"It had a lot of cushioning and I think that saved the baby. The photo looks like there's blood around it but that was just the tie-dyed colour of the blanket," he said.

"That baby had no chance if we and the other people hadn't been there. Something made us find that baby today."

Inspector David Lagats, from Quakers Hill police, said there would have been grave fears for the child's welfare if he had remained undiscovered for much longer.

The mercury soared above 45 degrees in parts of western Sydney as Sunday wore on.

"It's a horrific incident, but with all the teamwork from the bystanders, too, it was a good result and hopefully the child will survive," Inspector Lagats said.

Such cases "rare"

Department of Family and Community Services western Sydney district director Lisa Charet said such incidents were rare.

Post-natal depression, or mothers who did not know where to turn for help, had been factors in previous cases, she said.

"I think you have to be in a fairly desperate place to commit this sort of act," she said before the mother was located.

Andrew McCallum, from the Association of Children's Welfare Agencies, urged people to withhold judgment until the circumstances surrounding the incident were clear.

"And even then there's usually a lot of other factors involved in these things," Mr McCallum said.

Police said investigations were ongoing.