A couple have been sentenced to life in prison for murdering their five-year-old daughter three years ago in what a Hong Kong judge described as one of the worst cases of child abuse and a crime marked by “extreme cruelty”.

The husband and wife were also on Tuesday given concurrent jail terms for two counts of child cruelty, to which they had admitted, for the ill-treatment and neglect of the girl and her then eight-year-old brother. The children were both found with about 130 injuries after the girl died of septicaemia on January 6, 2018.

The 29-year-old father, a transport worker, and the 30-year-old stepmother, a housewife, each received 9½ years for child cruelty, just shy of the maximum 10 years.

The woman’s 56-year-old mother, an accounting clerk, was jailed for five years on two counts of the same cruelty charge, for neglecting the children while the five-month-long abuse was happening in her flat.

The case is believed to be the first instance of fatal child abuse that resulted in a murder conviction with a life sentence in the city.

The case was heard at the High Court in Admiralty.

Sentencing at the High Court, Mr Justice Albert Wong Sung-hau said the case was one of “extreme cruelty” involving both physical and psychological abuse inflicted upon the children for a prolonged period, with deliberate efforts made to conceal the “extensive and some very serious injuries” that he described as dreadful.

Wong said the step-grandmother was also guilty of serious neglect, noting she had failed to take the children to the doctor and had acted selfishly in a way that amounted to “acquiescence or connivance to the conducts of the other two defendants” when she might have been the siblings’ “only hope”.

“What [the parents] did on [the girl] ranks as one of the worst cases of its kind,” the judge said. “If [the grandparent] had not neglected the girl, her death could have been avoided.”

Wong also quoted from the Bible as he addressed the stepmother, who had identified as Christian.

“If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness,” he read from the Book of John.

But the stepmother did not react.

In the public gallery, a man shouted “savages”, while another cried out: “You should be ashamed of yourselves.”

Dozens of people arrived at the court building hours ahead of the afternoon hearing to queue for seats. The judiciary had earlier arranged for a bigger courtroom and after it filled up, the rest of the crowd was sent into a lobby where the proceedings were shown via live telecast.

Outside court, police displayed two pink slippers, a pair of scissors and a 47cm-long rattan stick that were used in assaulting the children.

The senior police officer whose unit handled the investigation welcomed the sentences, which she believed would act as a deterrent.

Chief Inspector Ko Mei-yee said corporal punishment was not an acceptable way to teach or punish children and parents should seek professional help if they found themselves in need. She also urged people to step forward if they knew of child abuse.

“A single report can protect children from further harm,” Ko said.

The trio were convicted by a High Court jury last week following a month-long trial filled with heartbreaking testimonies revealing how the siblings were punished, assaulted and deprived of basic life necessities such as food and access to professional medical treatment, even as their wounds festered and affected daily functions such as sitting and walking.

Derek Lai Kim-wah, senior assistant director of public prosecutions, said the chronic abuse was a significant cause of the girl’s death because it weakened her immune system’s ability to fight the salmonella infection that eventually killed her.

That was reflected by the experts’ finding of the change in the girl’s thymus – a vital organ responsible for the production of white blood cells that fight salmonella – which had been reduced to its smallest size, despite it generally being at its largest in children her age, in response to toxic stress.

The court also heard how their schools had noticed some of the signs of abuse. Their handling of the case has renewed debate on how children could be better protected and prompted the government to revise measures and guidelines, as well as to provide more social workers for schools.

The judge issued a gag order barring the identification of the family members and schools involved, to protect the siblings and their then seven-year-old stepsister, who was not abused.

In mitigation, lawyers for the couple had argued the abuse was “not the worst of its kind” given that it had happened in the course of disciplining the children.

They said the parents did not know how to seek outside support, while noting that there had also been “moments of joy” in the family.

But the judge countered on Tuesday: “Such episodes were just a few glimpses of consolation in the miserable period of life of the two children.”

Charity Save the Children Hong Kong said it was deeply concerned about the “horrific” case and noted the sentencing served as a reminder that child abuse would not be tolerated.

“We must do whatever we can as a community to ensure children’s safety and prevent future cases from occurring,” chief executive officer Carol Szeto said.

She urged all parties to offer long-term, holistic care and support to the boy to help him recover from the traumatic experience.

The group also called on the government to establish a mandatory reporting system for professionals who interact with children, outlaw corporal punishment and provide educational programmes on positive parenting.