Taking into account the time spent in custody, the 45-year-old woman who cannot be named due to a court order, was granted a non-parole period of 18 years and will be eligible for release in 2037.

The 20-month-old boy was found unresponsive at a property outside Dubbo, by paramedics on March 23, 2015, after the foster mother called triple-0.

The court heard the woman was unaware paramedics had answered the call when she was heard saying "they're going to think I bashed him", before asking "am I going to get in trouble for this?" when she called triple zero for help at 5.17am.

Despite multiple attempts to revive the child, he was pronounced dead at 7.30am at Coolah District Hospital.

During the trial the court heard investigations found the boy had injuries to his bottom area and extensive bruising to his abdomen and face.

It was also revealed he had suffered a stomach rupture and a broken femur - the break so severe, it was compared to that sustained in a car crash.

READ RELATED:

Ultimately the court found the child died as a result of the stomach perforation - which could only have occurred due to a "sustainable blow" by the woman a few hours before his death.

The foster mother, who has three biological children of her own had given multiple, varying accounts to police of what happened on the night of the child's death.

In January 2018 the woman was arrested in Geurie and charged with murder.

The former foster mother pleaded not guilty to one count of murder and spent almost four years on remand.

A 17-day judge-alone Supreme Court trial began in Dubbo February 2022, before Justice Richard Cavanagh who found the foster carer had "assaulted or attacked" the infant in the hours prior to his death.

The woman did not give evidence during the trial.

When handing down his sentence on Thursday, Justice Cavanagh said while he accepted the woman did not intend to kill the toddler, any explanation for such conduct afforded "no excuse".

"The killing of a defenceless child, even arising from a temporary loss of control without the intention to kill, deserves strong denunciation by this court," he said.

"The deceased died as a result of a violent assault perpetrated by the offender during which time he sustained the injuries.

"This assault arose as a result of a loss of control by the offender. Whether that was because the offender had been drinking, or she was particularly tired, or the deceased was behaving misbehaving is not known.

"Whatever the explanation for such conduct, it affords no excuse."

Justice Cavanagh said he was satisfied that the woman had intended to hurt the boy.

"An adult cannot punch a 20-month-old child with such force as to cause a stomach perforation without intending to cause grievous bodily harm," he said.

"An adult cannot impact such force on a child as to result in a complete fracture of the femur without intending to cause serious injury to the child."

Ultimately while he didn't accept the crown's case that the woman knew she had hurt him so badly he might die, he said her state of mind must have been that "she did not care".