The matter is being held in the Court of Criminal Appeal before Justices Gleeson, Lonergan and Button.

The child, who cannot be identified, died on April 2, 2014 after being in the care of Brendon Toohey, then 35.

During Friday's hearing, some of the baby's family left the court as details of the injuries became too much to bear.

Toohey has always maintained the baby suffered critical injuries when she fell from a trampoline and hit her head on the lid of a concrete septic tank as he was taking clothes off the washing line at a Mandurama home.

He was initially charged with the child's murder and, in 2017, was convicted of her manslaughter and was jailed for seven-and-a-half years by Justice Desmond Fagan with a non-parole period of four-and-a-half years.

He was eligible for release on October 8 last year.

In a separate, judge-alone trial in the Parramatta District Court in March this year before Judge J Bennett, Toohey was also convicted of the sexual assault of the same infant and was jailed for four years and six months (three years non-parole). The sentence was backdated to March 9, 2016, making Toohey eligible for parole the day after he was sentenced.

One month later, the Office of the Director of Public Prosecutions announced its intention to appeal the four-and-a-half-year jail sentence for the sexual assault on the grounds it was "manifestly inadequate".

In the Court of Criminal Appeal on Friday, crown prosecutor for the office of the Director of Public Prosecutions, barrister Guy Newton, submitted the facts in the present matter (sexual assault) drastically altered Toohey's intent and conduct.

Mr Newton said in all likelihood, the factual matrix on which Justice Fagan sentenced Toohey, which was later accepted by Judge Bennett, would have been altered had the second matter (sexual assault) been part of the first sentencing.

Mr Newton said Judge Bennett adopted the factual findings Justice Fagan had made, which he said significantly affected totality and accumulation of sentencing, saying effectively Toohey got "double benefits".

He said in sentencing, "Judge Bennett almost treated the sexual assault like an assault occasioning actual bodily harm, and his only reference in judgement as a sexual assault was the sentence being made would be wholly concurrent if not for the 'intimate' nature of the assault".

He also said Judge Bennett accepted expert evidence there had been forceful penetration of the child's anus by a blunt instrument, but "the highest factual finding we got [in sentencing] is some level of discomfort [to the victim]".

Mr Newton said Judge Bennett accepted Justice Fagan's findings the violence was not prolonged or with gratuitous cruelty, rather the result of an apparent loss of control and patience, and he submitted this clearly indicated the way in which His Honour erred in the determination to the extent sentencing should be concurrent.

Mr Newton said it was also his submission, given the evidence, it was difficult to see how a finding the sexual assault was not for sexual gratification was made.

Toohey's barrister James Trevallion submitted to the court there was no evidence that Toohey's motivation in the offence was anything other than "a response to duress and frustration; that was the finding His Honour made".

When asked by the court, regarding the injury of sexual assault, if he thought that was gratuitous or cruel, Mr Trevallion said "there is no evidence to suggest it was".

Asked if he accepted that striking a child in some places was considered more cruel than others, he conceded it was.

It was then put to him by the court that Justice Fagan had "blinkers on him" on the basis of one offence which was isolated.

"This sentencing judge knows of another offence, is not isolated, same timeframe, same period. Isn't it of a different character?

"It's not like a poke in the eye to a young child."

Mr Trevallion conceded it wasn't.

The father and paternal grandmother of the baby girl, who were among those attending the hearing, had welcomed news the DPP was appealing the sexual assault sentence.

At the time, the grandmother said they would not give up fighting for their little girl.

Judgement in the matter was reserved on Friday, and will be given in due course.