Only the parents of a two-month-old baby who died after being forcefully shaken know what happened on the morning of the infant's injuries.
But they aren't telling, a coroner says.
The infant was taken to a Sydney hospital suffering cardiac arrest and haemorrhages on May, 11, 2009, after his father called triple 0 from their home.
An inquest heard the baby's father claimed he picked up his son, who was born at 28 weeks, and the baby vomited, choked and lost consciousness, prompting the father to call an ambulance.
The mother claimed she left the home just before she dropped her daughter to school.
However, Deputy State Coroner Elaine Truscott stated there had been too many "lies, contradictions and inconsistencies" from the parents.
"Only the parents know the true events of the morning of 11 May 2009, and neither has told it," she said in written findings released on Friday.
The baby died a month later from a "catastrophic" brain injury.
Evidence to the inquest showed the baby had injuries consistent with being picked up or thrown down aggressively.
There was a submission the mother had cognitive difficulties but Ms Truscott didn't believe that could stop her from colluding with the father to stop the truth coming out.
The coroner found a jury could not be satisfied with the guilt of one parent at the exclusion of the other.
The case raised questions over how adequate the law was in dealing with cases where it's not possible to distinguish who - out of two people home when a child dies - was responsible for a child's non-accidental death, Ms Truscott found.
In 2013 the NSW attorney-general was asked to consider law reform so a perpetrator in such cases, could be brought to justice.
Ms Truscott said she would forward this case to the attorney-general and recommend his consideration of the law change going ahead.
She referred to a research paper showing an average of eight people a year are killed by a family member, primarily a parent.
"I do not know how many of those perpetrators are not brought to justice because of the difficulty surrounding criminal liability," she said.
Ms Truscott found the baby died after being shaken by a person or persons unknown and referred the case to the unsolved homicide team.
A spokesperson for the NSW attorney-general said deliberately injuring children was an extremely serious matter.
"The attorney-general will look at all recommendations made by the coroner and take advice on the complex issues involved," the spokesperson said.
