An Australian mother was Monday charged with attempted murder after her newborn baby was found crying at the bottom of a roadside drain, where investigators believe he survived for five days.

Passing cyclists heard wailing coming from the 2.5metre (8.2 feet) deep drain on a bike path along a western Sydney highway early Sunday morning.

After several people lifted a heavy concrete slab, the baby boy -- wrapped in a striped hospital blanket with his umbilical cord cut and clamped -- was found at the bottom.

Police on Monday charged a 30-year-old woman with attempted murder after spending several hours searching hospital records and knocking on doors.

They revealed the boy had been in the dirty drain since last Tuesday, likely squeezed through a small gap before plunging to the bottom.

"A woman has been charged with attempted murder after a newborn baby was found in a drain in Sydney's west on Sunday," police said in a statement.

"Police will allege the baby, believed to have been born on Monday, was placed into the drain on Tuesday, November 18."

The baby remains in a serious but stable condition.

"It was disturbing (to see) how the child was placed in the drain," Quakers Hill police Inspector David Lagats told reporters on Sunday.

"We all thought the worst but the baby was still alive."

"We actually thought it was a kitten at first, but when we went down there we could hear exactly what it was, you could definitely tell it was a baby screaming," David Otte, one of the cyclists, told The Daily Telegraph.

"We're just thinking about the little fella, he's a beautiful, beautiful baby."

Police doubt the baby would have survived much longer as temperatures in Sydney's northwest reached 40 degrees Celsius (104 Fahrenheit) 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," Lagats said.

The baby is now in the care of the New South Wales Family and Community Services, and it will be up to the agency to assess what now happens to him.