A neighbour, who asked not to be named, said the parents called her over to take a look at the baby as an ambulance was being called.
The woman arrived to find the baby's eyes rolling in his head "and he started going a bit purple".
The woman said the baby's temperature was normal and she couldn't see any signs of injury, bruising or bleeding.
"But he couldn't breathe properly from what I could see, he was gasping for air and his father was on the phone to the ambulance."
The neighbour, who said the baby was a premature twin and had been sickly most of his life, said she heard the St John operator saying the baby might have had measles.
A St John spokesman later confirmed that the baby had head injuries and was taken to hospital in a critical condition.
Mr Greene said the York St address where the baby was found was under guard and would be the subject of a scene examination by forensic experts today.