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Police are investigating whether the 4-month-old baby admitted to hospital with head trauma had suffered any other injuries.
The Papakura boy was taken to hospital last weekend, and remains in a critical condition at the Starship's high-dependency unit.
Police are investigating how he got his injuries, which required surgery.
The boy was suffering from head trauma when he was admitted to hospital late last Saturday but the Weekend Herald understands doctors may have also found evidence of older injuries.
The boy's 18-month-old brother has been taken into the custody of Child, Youth and Family.
Police spokeswoman Denise Traill said it was standard practice for police to look at whether there was any previous history.
That included looking at medical records and talking to the family. The baby's teenage parents are said to be co-operating with police.
Ms Traill said eight officers from the child abuse team were working on the inquiry.
Forensic staff and other officers had canvassed the area surrounding the baby's home which had been cordoned off until Thursday.
Neighbours say they know little of the occupants who have lived in the Housing New Zealand home for at least four years.
But they told of frequent parties being held on weeknights that frequently end in yelling and screaming.
"They can be singing along to the music then whammo, something triggers them off," said one man.
"There was one time when I thought, 'Boy, it won't be long until someone's going to get hurt."'
The man had seen the toddler and heard a baby crying in recent weeks. The children lived at the home with their parents and maternal grandmother, but other people were always coming and going.
Neighbours say there have been about three fires at the house in the past few years, including one in which a Herald delivery person saw smoke coming from the windows and alerted the family.
A resident in the street said police arrived at the house last year in what is believed to have been a drug raid. Four occupants were taken away for questioning but it is not clear if they were arrested.
No one from the family was available for comment last night.
