When David Otte set out for a Sunday morning cycle, he never imagined he would find a newborn baby boy trapped down a drain.

He was riding along the M7 bike track at Quakers Hill about 7.30am with his 18-year-old daughter Hayley when they were stopped by two men who could hear noise coming from a drain.

The men thought it was an animal - a cat or a lizard, they said - but Mr Otte knew better.

"I've got two kids of my own so I know what a baby screaming sounds like," the father from Rydalmere said. "It was so intense; you couldn't not tell it was a baby. We couldn't see it but we could hear it. It was distressed."

They called police immediately and "desperately tried to lift the drain cover".

"But it probably weighed a tonne," he said. "There was no way three guys and a young girl could lift that. It was huge. It was probably 11 or 12 centimetres thick."

When police arrived, it took seven people to lift the concrete lid.

"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."

Police said the baby boy was likely to have been pushed through the narrow drain opening before falling about 2.5 metres.

He appeared to have no external injuries but was malnourished. He is being cared for at The Children's Hospital at Westmead and is in a serious but stable condition.

The baby is thought to be of Indian or Middle Eastern background and was likely to have been born within the past two or three days.

Police said there was no indication of how long the baby had been there.

The baby's umbilical cord had been cut and clamped and he was wrapped in striped blankets similar to those issued in hospitals.

Mr Otte said the baby also 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. "It's hot out there now. That baby wouldn't have lasted an hour and a half. That baby was brand new."

He said it was "just unbelievable" to come across the abandoned baby.

"You go through life seeing things but you never ever imagine you'll see something like this. That baby had no chance if we and the other people hadn't of been there. Something made us find that baby today."

Police said a man of Indian appearance wearing an orange shirt was seen leaving the scene.

Anyone with any information is urged to contact police.