UPDATED 5.20pm: Police have found the mother of a newborn baby discovered 2.5 metres down a drain near a Sydney highway.

Cyclists on a bike track beside the M7 at Quakers Hill heard cries coming from the drain around 7.30am today.

With the help of passers-by and police, they lifted a large concrete slab that covered a stormwater pit and discovered the baby wrapped in a hospital blanket.

Believed to be two or three days old, he's in a serious but stable condition at Westmead Children's Hospital.