The seven-year-old boy who was abused by his mother and her boyfriend.

A Sydney mother who pleaded guilty to the manslaughter of her seven-year-old son has been sentenced to at least 10 and a half years jail after admitting he was subjected to months of neglect and sickening physical abuse at the hands of herself and her boyfriend.

The couple originally told paramedics and police the boy had fallen off a pogo stick and hit his head. Months later, the woman admitted to detectives her boyfriend instructed her to lie because he feared being charged with murder.

On Thursday the NSW Supreme Court heard the 27-year-old woman, who cannot be named for legal reasons, had been a responsible and attentive mother until she began a relationship with a Christian hip-hop music producer.

Justice Ian Harrison said she adopted his "extreme religious views" and became influenced by him, particularly his "parenting philosophy", upon starting a relationship in January 2013.

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The judge accepted opinions by forensic psychiatrists who likened the couple's relationship to Stockholm Syndrome or a cult, as the mother surrendered all decisions about her children's care and treatment to him. She even took part in a sickening regime of corporal punishment, abuse and neglect that continued until the boy's death on May 21, 2013.

Justice Harrison said in failing to seek medical attention for her son, who had a moderate intellectual disability, after he suffered a serious head injury, "she has breached the most fundamental trust between human beings".

Her decision to go about her daily activities despite her child being unresponsive and failing to improve over several hours was "profound, cruel and selfish".

However, the judge accepted the mother was genuinely remorseful and gave a discount of 40 per cent for pleading guilty and for agreeing to give evidence against her former partner.

He sentenced her to a maximum 14 years jail with a non-parole period of 10 and a half years.

She showed no emotion as the sentence was handed down.

The man, who cannot be named for legal reasons, has been charged with murder and will stand trial later this year.

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