The newborn girl whose body was found on a beach in Melbourne's south-east had been left there for about two days.

Days after the discovery of the remains in Seaford, police do not know how the baby died or who her mother is.

Testing has revealed the baby was probably left on the beach near the Nepean Highway sometime between 7am and 5pm Monday. Her remains were found by a passerby about 4.30pm Wednesday.

She had her placenta and umbilical cord still attached. Police would not say whether the baby had been injured.