NSW Police have serious concerns for the wellbeing of a newborn baby and its mother after finding evidence the child was born on the banks of the Cooks River in Sydney.

Emergency services were called to the river bank near Earlwood on Monday afternoon about 4.30pm after receiving reports that the remains of a birth were found.

The ABC understands they found a placenta.

Detectives have established a crime scene and a widespread search was undertaken around Wardell Road in Earlwood.

An investigation has been launched and sources have told the ABC that police are worried the baby could be in the water.

They are urging the mother to attend her nearest hospital to receive urgent care for her and her child.

Police are appealing for information and anyone with CCTV footage is urged to contact Burwood Police or Crime Stoppers.