A woman ran to her neighbour's house to find a critically ill baby going purple and his eyes rolling in his head yesterday.
The baby was then rushed to Waikato Hospital but had to be airlifted to the Starship children's hospital with head injuries following what police say was a domestic-related incident.
The Waikato field crime manager, Acting Detective Inspector Mark Greene, said police had arrested a 32-year-old man.
He will appear in the Hamilton District Court today to face a charge of assault on a child.
The baby boy, understood to be about 9 months old, was taken to hospital by ambulance at 11.35am after the incident at a York St home in Hamilton East.
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.