A two-year-old boy from Sydney's north-west could have sustained critical head injuries up to two days before he was taken to hospital by his parents, court documents suggest.
The North Rocks boy was taken to Westmead Children's Hospital at 8:30pm last Wednesday with head and body injuries, which were described as "traumatic" by Child Abuse Squad detectives.
The boy's father has been charged with recklessly causing grievous bodily harm with intent.
The boy's 24-year-old mother has now been arrested and charged with recklessly inflicting grievous bodily harm with intent and hindering a police investigation.
The woman was released without charge on Friday, but she was arrested again earlier today outside Parramatta Police Station.
It is alleged she witnessed the attack and did nothing.
Police said his parents told them the child had fallen down a set of stairs twice in three days, and blamed some of the injuries on the boy's three-year-old brother.
Neither the boy nor his parents can be identified for legal reasons.
The boy's father appeared in Parramatta Local Court on Saturday and again today.
He was refused bail on both occasions to appear again on October 2.
Court documents reveal police could only establish the toddler was injured between 12:01am on Monday, July 28 and 8:30pm the following Wednesday, indicating he could have been badly injured for up to two days before his parents took him to hospital.
Police told the ABC the boy's injuries were extremely severe.
They said the family was known to the NSW Department of Family and Community Services (FACS).
The ABC understands the boy and his brother are now in the custody of FACS.
FACS confirmed the department was aware of the case and the child remained at Westmead Hospital with serious injuries.
The boy's mother was refused bail and will reappear in court on August 14.