KEY POINTS:
Police say someone is being "economical with the truth" after a 4-month-old child suffered a serious head injury over Labour weekend.
The baby boy, from Motueka near Nelson, was initially taken to Auckland's Starship hospital in a critical condition with a fractured skull.
He has since improved to being in a serious condition in the paediatric intensive care unit, and is expected to survive.
His parents called an ambulance on Monday night.
Detective Sergeant Kevin Tiernan told the Weekend Herald the injury was consistent with the child being banged against an object and "probably more than that, in terms of shaking or something like that".
"The severity of it confirms that it's non-accidental."
The child was being cared for by his parents over Labour weekend when the injury is believed to have occurred.
"We are unable to identify a particular incident or time when this event could have occurred," Mr Tiernan said.
"The injury may have happened a day before and hasn't manifested until a day later."
The parents and several others who had been in contact with the baby over Labour weekend have been interviewed by police.
"Everyone is being fully co-operative ... but someone is being economical with the truth."
Mr Tiernan said it was possible the child could suffer long-term consequences from the injury.
The parents are in Auckland while their child receives treatment.
Child Youth and Family (CYF) spokeswoman Helen Aiken said the public could be reassured that the baby boy's safety and wellbeing "is now our responsibility".
The 12-month-old child of an extended family member was also in the care of the parents, but is also now in the care of CYF.
Mr Tiernan said police would consider further inquiries, and may re-interview people of interest.
