Police in Australia have questioned but not charged a 30-year-old woman whose newborn baby boy was found abandoned today in a drain near a Sydney motorway.

The baby, who is only a few days old, is in a serious but stable condition in Sydney's Westmead Children's Hospital, police said in a statement.

The mother was taken to Blacktown Hospital in Sydney after being questioned by detectives, police said. There were no details released on her medical condition.

A group of cyclists were riding on a cycle lane beside the M7 motorway in the Sydney suburb of Quakers Hill when they heard a baby's cry coming from the drain, police said.

They managed to lift the heavy concrete lid off the drain, which is 8ft deep, and saw the baby wrapped in a hospital blanket at the bottom.

"We actually thought it was some kitten or something like that," cyclist David Otte told Ten Network television news.

"It was only when he went down there and had a real good look at it that we could hear exactly what it was."

Police reached the scene soon after and rescued the child.

"It was disturbing how the child was placed in the drain," said Quakers Hill Police Inspector David Lagats, who estimated the baby was two to three days old.

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

The boy had no signs of physical injury but was malnourished and dehydrated, Mr Lagats said.

Police said they found the mother today by checking hospital records and knocking on doors of homes in the area.

They did not release the mother's name but said she was from Quakers Hill. She was taken for questioning.

Police suspect the baby was squeezed through the drain's narrow opening and dropped to the bottom.

They did not say how long the child had likely been left there. There has been no rain in Sydney for several days, so the drains are dry. But rain was forecast for tonight.