A mother who killed her three-year-old son "because he looked like his father" has been sentenced to at least 33 years in jail, while her partner has been given a prison sentence of at least 30 years.

Key points: Justice Peter Johnson said the dead boy had been worn down by "torture"

Justice Peter Johnson said the dead boy had been worn down by "torture" The case traumatised police officers, who later quit the force

The case traumatised police officers, who later quit the force Outside court, a family member said the dead boy's mother had shown "no remorse"

The 43-year-old woman and 48-year-old man, who cannot be named for legal reasons, were found guilty of murder in the New South Wales Supreme Court in April.

The toddler, known as Joseph, died from extensive and horrific injuries to his head and body after months of violent abuse in the town of Oberon in New South Wales' central west.

WARNING: This story contains graphic details which may distress some readers.

The court was told the mother admitted to police that she had thoughts about killing her son because she "just didn't connect with him".

Among the abuses Joseph suffered at the hands of his mother and step-father, his eyes were taped closed with duct tape because he was staring at them.

He had been hit with a spoon and, in one instance, forced into an esky for ice baths in a bid to bring out his bruises.

In the days before his death, Joseph's mother — referred to during the trial as LN — jammed his head in a wardrobe door and punched him in the head.

"Don't get me wrong I did love that boy ... but there was a part of me that hated him ... because he looked like his father, because he's his father's child," she told police.

Boy was worn down by 'torture'

Joseph died from extensive and horrific injuries to his head and body after months of violent abuse. ( Supplied )

Outside court, Detective Sergeant Joel Fawkner from NSW Police said officers had quit the force after being involved with the "dreadful" case.

"It's very traumatic, I think this was inhumane and the torture of acts upon Joseph — you can't unsee what we've seen," he said.

In sentencing, Justice Peter Johnson said the child's mother and step-father — referred to during the trial as AW — had worn Joseph down.

"Rather than nurturing her son, LN punished him as if he was in some way responsible for the sins of his father. This was a grotesque and cruel feature of LN's conduct towards her son," he said.

Justice Johnson said Joseph was vulnerable.

"By the time of the final assault causing death I am satisfied that Joseph had been worn down physically and psychologically by the assaults upon him so as to render him more vulnerable to the final and fatal attack," he said.

Extended family praised for their compassion

Speaking to Joseph's extended family who were in court, Justice Johnson told them it was not their fault.

"In my view the extended family members should not in any way blame themselves for what happened," he said.

"It is a measure of their decency and compassion that they have reacted to these events in this way.

"Each of them provided care, love and support to Joseph while he lived with them in Sydney for several years."

Justice Johnson said the jail terms were "de facto life sentences".

"If Joseph had continued to be raised with the extended family in Sydney in 2014 there is every reason to believe he would have grown into a fine young man," he said.

The toddler's 48-year-old stepfather was jailed for at least 30 years for his murder. ( Supplied: NSW Police )

Outside court, the dead child's uncle said he was pleased with the sentences.

"The judge dished out the appropriate punishment, as far as I can see," he said.

The uncle said he did not believe the child's mother and her partner had an understanding of what they had done.

"All through the court process, I haven't seen any remorse or any behaviour that would suggest they were upset by this whole ordeal," he said.