The man, 34, who cannot be named because it will identify the young girls, was jailed in Newcastle Supreme Court last year for a maximum of 37-and-a-half years, with a non-parole period of 28 years after he pleaded guilty to murdering his 12-year-old step-daughter during a three-day beating in September, 2015.

The girls' mother pleaded guilty to manslaughter on the basis of gross criminal negligence and last year was jailed for a maximum of four years, with a non-parole period of 18 months.

But a few months after the pair were sentenced, detectives revealed what those close to the case had known since the man's arrest: the young girl who died was not the only child in the house being subjected to heinous acts, involving "gratuitous cruelty to a young, helpless and defeated victim".

As well as witnessing the brutal assaults inflicted on her 12-year-old sister, the man's other step-daughter was also being subjected to savage and sustained beatings over the same period, spanning a number of years until her sister was found dead in her bed in 2015.

The girl, who was 10 when her sister died, had been tied to the bed and beaten, hit over the head with a metal spoon with such force that the spoon bent, thrown to the ground and had her arm broken, punched in the stomach until her ribs fractured and hit with a power cord among other savage forms of torture, according to an agreed statement of facts.

On Monday in Newcastle District Court, the girls' step-father, who pleaded guilty to causing grievous bodily harm with intent and two counts of assault occasioning actual bodily harm in relation to the assaults on the surviving sister, was jailed for a maximum of nine years, with a non-parole period of six years, partially accumulated with the murder sentence he is already serving.

He is now eligible for parole in 2046, at the age of 62, when he will be deported to his homeland in Africa.