The 45-year-old woman was found guilty after a 17-day judge alone Supreme Court trial, which oversaw evidence presented from 26 witnesses and nine expert medical witnesses.

Justice Richard Cavanagh found the foster carer had "assaulted or attacked" the infant in the hours prior to his death.

It was the Crown's circumstantial case that the woman had assaulted the boy at the home sometime in the night of March 22 and the early morning of March 23, and that she had intended to inflict grievous bodily harm.

They argued the injuries the boy was suffering from at the time of his death were such that they could have only been caused by a deliberate assault by the woman.

The woman, who become a specialist carer with Uniting Care Burnside, had taken the boy in to live with her in November 2014, along with her partner and three biological children.

The court heard the woman was hoping to have him in a long-term placement, but discovered the child was going to be placed in the care of a family looking after his two half brothers.

In the lead up to the infant's death, the child had spent the last two weekends with his future foster parents and half siblings, including two days prior to his death.

The family at the centre of the murder had attended a gathering at the accused's brother-in-laws, on the evening of little boy's death.

The court heard up to 22 neighbours and children gathered at a property to watch rugby league. The foster carer told police she consumed five to six beers that night.

Witnesses at the barbecue said nothing unusual happened, and the boy was seen to be walking, running, playing and feeding during the afternoon and evening.

One witness also gave evidence that the child had gone to sit on her lap and was grinning and trying to dance by bopping his head.

While some noted that he seemed a little subdued, the accused explained that he had been sick.

The time the family returned home was uncertain but around 8.30pm and 9.30pm.

About 5.17am, the foster carer called triple-0 and was on the phone to the operator for 42 minutes.

The court heard the woman was unaware paramedics had answered the call when she was heard saying to her partner, "they're going to think I bashed him".

The initial description provided by the woman to the operator was that she had a little 18-month-old foster child who had fallen over in the bath the night before.

She said that she had heard him making noises during the night, and that she now thought he had thrown up and "swallowed it again". She also said "now he's not breathing very good" and that she had "just tried to give him CPR and now he's like he's full of air".

When asked if the boy was conscious she said, "No his eyes are open ..." but her partner interrupted and said, "he's not breathing".

She was later recorded telling the operator the boy had bruises from the bath and falling over and hitting his head, before asking "am I going to get into trouble for this?".

It took paramedics 40 minutes to arrive at the property. Two teams of paramedics attended and when the first team arrived saw the woman was attempting CPR on the boy.

Ambulance officers who arrived at the home at 6am that morning had noticed bruising to the child's face and that he had a distended stomach.

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

An autopsy was conducted on the child by Professor Timothy Lyons from the Department of Forensic Medicine at Newcastle on March 25, 2015.

Professor Lyons concluded that the deceased child was suffering from multiple injuries, which included injuries to the bottom area, extensive bruising to his abdomen and head.

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

Professor Lyons reported that in his opinion the severity and number of injuries was inconsistent with the injuries being "accidental in nature".

The former foster carer didn't give evidence during the Supreme Court trial, but the court heard evidence from her three police interviews.

In the three interviews she gave varying accounts to police of what happened on the night of the child's death.

In her first interview with police three hours after the child's death on March 23, 2015, her behaviour was described as "bizarre", in that she was smiling and making jokes.

During the interview the foster carer claimed the boy had behaviours such as "head butting things" and was "always hurting himself". She also said the infant hated the bath.

The woman told police the boy had a "bit of gastro" the week before his death, and when he returned home on the Saturday he was "still pretty hot".

She told police after returning from watching the football, she changed the boy and put him to bed.

However, she told officers he had thrown up in the cot so she took him for a bath, which is where she claimed be began rolling around and hit his head, and swallowed a bit of water.

About 10pm she said she put him to bed, and about 2am or 3am heard the child making some noises.

The next morning her partner was leaving to go to work when she woke up and checked on the boy, who "didn't look right".

She tried to sit him up, but he was "floppy". She said that "he was still breathing, but not good," before her partner rang triple-0.

In her second four-hour interview with police on April 1, 2015 at Mudgee Police Station, the woman was questioned about the boy's injuries.

Police asked her if at any point she had dropped the infant when she responded "I could have".

"No, no, I don't think so ... I remember shaking him ... and grabbing him. I might even grabbed him by the head in the cot ... I think I grabbed him by the head, by the head, face by the hands, body, just, got him, put him on the floor right there," she told police.

Towards the end of the interview, the woman was informed of the injuries which had been found on post-mortem, including the fractured leg.

She said it might have occurred when she "panicked" and shook him around when she got him out of the cot.

When further pressed for an explanation, she said, "Even when I got him out of the cot I must have chucked him around or forced him or threw him harder than what I thought. I was in a panic. That's the only way I can explain it. I don't know. Like I said I grabbed him ... I don't know I panicked ... I didn't want to hurt him."

When giving judgement on Tuesday, Justice Cavanagh said he did not take this interview as admission to breaking the boy's leg, but more of a statement that she couldn't explain it and there was no one else who could have done it.

On June 23, 2015 the woman contacted police and invited them to her house for a further interview to show officers the home, and explain some of bruising.

She said the boy would "just head-butt things, like unbelievably"; she had never seen a kid like that and he didn't care or show it if it hurt him.

She said he hated tea time, he hated bedtime, he hated bath time, he hated restrictions.

The woman then went to various places around the house demonstrating where he would head-butt the pole or the wall. She said that other people had seen him head-butt, such as her sister and the caseworker.

When asked if she knew how the boy might have broken his leg, she said the only way to explain it would be that her partner might have trod on him when he was going to see if the ambulance was coming.

The officers pressed as to whether she could remember it happening, she said she remembered him getting up but she could not remember the detail of it, her brain just wouldn't let her.

The court was told the woman claimed the boy's injuries could have been self-inflicted or inflicted by another person.

After being interviewed separately on April 1, the woman and her partner were recorded driving home on a secret listening device.

In the recording, the woman was "frustrated and angry" about what had happened at the police station, but said nothing that could be considered as any sort of admission.

At one point however she revealed she did not tell police that one of the mothers at the barbecue had told her child not to be rough, and should not abuse the little boy. She said she didn't want to get him involved.

Justice Cavanagh said it was difficult to understand why she was trying to protect them, given she had told police the boy was fine when he got home and later threw up in his cot.

As events transpired the mother and boy agreed a conversation took place, but neither said anything in their evidence that the child had hurt the toddler in any way.

When giving judgement Justice Cavanagh said he believed the woman had either left things that happened in her home during the night, or provided information to police things that weren't correct.

The court revealed the woman's then 13-year-old daughter had witnessed the boy in the bath when she went to use the bathroom, but was instructed by the foster carer not to mention that to police.

The daughter told police her mother had been drinking that night and told them this is when "she can get aggressive".

She told the officers when she went to the bathroom she saw her mother yelling at the boy.

"I saw her aggressive at him, like, towards him, and when he was standing she just sort of, like, pushed him to be sitting, a bit rough," she said in her interview.

Justice Cavanagh said the foster mother omitting this suggested something else happened in the bath, that the woman did not want the police to know.

Justice Cavanagh said while no person called to give evidence had witnessed the woman doing anything to the boy "that might have led to the injuries which he was subsequently discovered to have suffered", the woman was the only person likely to have "assaulted or attacked" the boy.

He ultimately found that the woman's explanation of what the boy did in the bath did not explain the presence of various facial bruises, abrasions and scratches, including on his neck.

He said it also didn't explain some of the bruises found on his body, and provided no explanation for the fractured femur or stomach perforation.

"The accused had first aid qualifications. It is one thing to be shocked and panicked and perhaps lift a toddler up and try and move him around a bit. It is another thing to deliberately throw a 20-month-old child on the ground with force because of such shock," he said.

"The accused's version of how the deceased might have been injured overnight in her home is, according to the experts, not consistent with the injuries sustained."

Justice Cavanagh said it came to the "inescapable" conclusion that the boy had sustained the injuries in the home prior to his death.

He said while it may not have been planned, he accepted the woman deliberately struck the boy on one or more occasions and did it with the intent of causing grievous bodily harm.

"Whether she was affected by alcohol is irrelevant. The force required to cause the injuries would have been significant," he said.

"The femur and stomach injuries could not have been caused by some accidental excessive gripping or inadvertent twisting. Whether or not the actions of the accused arose out of a temporary loss of control such acts by an adult on a 20 month old toddler could only have been intended to cause grievous bodily harm."

The woman was taken into custody and a she will be sentenced in May.