William Andrew O’Sullivan was beaten and kicked by two inmates while in custody over the death of 22-month-old Mason Jet Lee.
He was knocked out and spent 13 days in hospital with a fractured skull.
O’Sullivan was last week jailed for nine years for the manslaughter of Mason at an Australian court.
His stepdad beat him so severely his small intestine burst – then simply looked on as his organs shut down, his stomach swelled up and his nappies became blood-soaked.
The little boy died alone, covered in vomit, on a bedroom floor at his killer’s home in June 2016.
He pleaded guilty to manslaughter and child cruelty and was sentenced to nine years behind bars on August 30, with a six-year non-parole period.