Superintendent Christine McDonald said lights were brought in to allow the launch of a large-scale search on Monday night.
“Obviously our search will hopefully identify what has taken place; whether the delivery of that child occurred at this location or another location is yet to be determined.
“I ask anyone with information to contact Crime Stoppers.
“It’s very important and it’s extreme urgency we find the mother and the child as soon as we can. We ask she goes to a hospital. We will be contacting hospitals.”
The Cooks River flows from Yagoona in Sydney’s outer southwest and runs into Botany Bay at Kyeemagh, 23km away. It is heavily polluted as a result of traffic congestion, litter, sewage, illegal dumping and industrial and domestic activities.