After his daughter was rushed to hospital, a man told police he knew he would be blamed for her severe injuries.
Those police tried to make it seem that way, but he was cleared following a judge-alone trial.
"The conduct of the police was at times, to say the least, unprofessional and improper," Judge Pauline David said on Friday.
"It was totally appalling at times," she said.
Police intentionally lied to the man, trying to trick him into saying something they could argue motivated him to hurt his own daughter, the judge said.
The baby's mother told the court she was listening through a monitor as the man put the baby on a change table, before she took a deep breath and went limp.
At a nearby emergency room, doctors found the baby had severe brain and spinal injuries.
He pleaded not guilty to causing grievous bodily harm during a judge-alone trial in the NSW District Court.
The "deliberate and grossly improper tactics" included police suggesting the baby was not biologically his.
He had no doubt the baby was.
The baby looked just like him, and another, older child he had with the baby's mother.
The judge also independently verified the man's assertion he was not capable of smacking his children and appeared shocked when asked about him committing any physical abuse.
But the mother had demonstrated a capacity to convincingly lie if it suited her interest, the judge said, only conceding when presented with incontrovertible evidence of the truth as she gave evidence over six days.
A significant portion of that time related to her diverse and complex mental health issues.
It formed part of the evidence that led the judge to conclude she was frequently unable to cope and was prone to anger, frustration and lashing out.
There were also numerous text messages.
"I don't propose to read them all but that's a sample," Judge David said, quoting messages from the mother describing a child as a "dumb c***" she was "f***ing sick of".
There was no evidence the mother inflicted the baby's injuries.
"If she had … I do not expect that (she) would admit it to either the doctors, police, or this court," the judge said.
There was also no direct evidence the man inflicted the injuries, and the prosecution's circumstantial case did not persuade the judge.
He did not have the medical expertise to opine on the baby's condition and whether it could be due to a disease, and rather than lying, may have instead displayed the wishful thinking of a concerned parent who found their baby's sudden deterioration inexplicable.
"It was borne of a consciousness of his own innocence, as opposed to a consciousness of his own guilt," the judge said.
A doctor told the court it was reasonably possible the injuries were inflicted as early as a day before the man attended to the baby on the morning she was taken to hospital.
Prosecutors portrayed the man as having a short fuse and operating under financial and familial pressures, which the judge did not find was established during the trial.
The improper interview tactics had the opposite of their intended effect, instead solidifying the judge's opinion the man displayed a "remarkable" level of forbearance and temperance in his relationship with his children and their mother, as well as police.
There remains perplexing issues about the case but prosecutors were unable to establish the man inflicted the injuries.
"I find you not guilty," Judge David told the man.
"That concludes the matter."
1800 RESPECT (1800 737 732)
Lifeline 13 11 14
beyondblue 1300 22 4636
