Police are searching for a mother and newborn baby after finding evidence she may have given birth by a Sydney riverbank.

Emergency services were called to Cooks River at Earlwood in the city’s south-west on Monday afternoon after a resident walking his dog found what police believed to be a placenta and umbilical cord.

Police say tests on the organs revealed them to be human.

Serious concerns are held for the welfare of the child and mother and detectives have established a crime scene, and launched an investigation into the incident.

Police urged the mother to attend her nearest hospital to receive urgent care.

Supt 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,” she said.

“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 south-west and runs into Botany Bay at Kyeemagh, 23km away.