Police believe a newborn baby girl found dead at a beach in Melbourne's south-east on Wednesday had been there since Monday afternoon.

A passer-by found the baby's body along with the umbilical cord and placenta at Seaford Beach about 4:30pm on Wednesday.

In a brief statement today, Victoria Police said investigators believed the baby was likely left at the beach sometime between 7:00am and 5:00pm on Monday.

"Investigators are continuing to search for the mother of the infant and hold serious concerns for her health and welfare," the statement said.

"They are appealing for the woman to contact police as soon as possible and want to reiterate that her wellbeing is the primary focus of the investigation."

On Thursday, police said they were yet to identify any of the people involved.

Anyone with information is urged to contact Crime Stoppers on 1800 333 000.