Detectives are investigating the death of a baby boy who was pulled unconscious from a bath tub at a home in Sydney’s south-west.

NSW Police were following up a concern for welfare report just before 5:00pm yesterday when they made the grim discovery at a property in Oran Park.

Sources close to the investigation said the four-month-old boy was found unconscious in the bath, with his mother lying on the floor.

Paramedics performed CPR on the child before he was taken in an ambulance to Campbelltown Hospital in a critical condition.

The baby died a short time later.

His 32-year-old mother was arrested at the home and taken to Narellan Police Station, where she was questioned by detectives.

The mother of two has since been taken to Liverpool Hospital for a mental health assessment and remains under police guard.

The Oran Park home remains a crime scene and detectives have begun door knocking neighbours as they continue to investigate the circumstances of the boy’s death.

-ABC