Police are continuing their investigation into what happened at the home on Sunnyside Cres, in Papatoetoe, after the discovery yesterday morning.
A police spokeswoman said early today that the injured child remains in a critical condition at Starship Children's Hospital in Auckland.
Neighbours reported seeing him being carried out in a stretcher earlier.
Exactly what happened at the house is not yet clear; only that the bodies of a man and a woman were found after police were called just before 10am.
Police would only say it was a "tragic event" and that they were working to piece together what had happened.
The woman's body was discovered in a bedroom. The child - whose age has not been released - was found in the same room.
Police found the man's body in the garage next door.
Authorities cordoned off the area and officers in white jumpsuits were soon examining the scene.
The adults' relationship to the child is not yet known.
Shocked neighbours yesterday gathered outside the house to talk and comfort each other, as police officers stood guard and worked in the background.
Residents told the New Zealand Herald the victims had been home last week and remembered the boy, whom they described as "full-on" - playing outside with toys he had received for Christmas.
"He got a new bike," one woman said.
Yesterday, the woman said she saw the boy being taken away. He seemed to have lacerations to his head.
"He looked lifeless," she said.
Authorities were yesterday working to confirm the victims' identities and to contact next of kin.