Porritt, 24, walked out of the ACT Supreme Court with his father and one of his sisters yesterday after Chief Justice Terence Higgins released him with a good-behaviour order after he was found guilty of the manslaughter of his mother in December 2005. If he breaches the order, he faces another three years' jail.

Nanette ''Sandy'' Porritt's New Zealand family are furious that Porritt walked free after spending just 22 months in custody for her death. Mrs Porritt's sister, Cathy Johnston, remains convinced Porritt murdered her, rejecting his story to the court that his mother had first attacked him with a knife.

''We're gutted, absolutely gutted,'' Mrs Johnston told The Canberra Times last night from her home in the North Island town of Maketu.

''To me, all her life and that justice system is worth is 22 months. That's what they deemed her life to be worth, as far as I can see. If I was in Canberra I'd be a little bit scared, because it shows you can get away with murder.''

Chief Justice Higgins had deliberated for almost two months about what sentence to impose. In explaining why Porritt was ready for release, the judge pointed to Porritt's otherwise good record and the strong family support he still had from his father, Keith, and younger sisters, Jenna and Amy. Amy is studying journalism at university in Wollongong.

Chief Justice Higgins said the punishment needed to reflect the offender's level of responsibility.

''It is impossible to reflect adequately in any sentence the enormity of the loss of the life of Nanette 'Sandy' Porritt.

''It is, however, the court's duty to punish any person bearing criminal responsibility for this event commensurately with that responsibility, after taking account of the personal circumstances of that person and the interest of the community in the rehabilitation and humane treatment of offenders.''

In April, two months after a trial before him sitting alone, Chief Justice Higgins acquitted Porritt of murder but found him guilty of the 50-year-old CSIRO lab technician's manslaughter.

The trial heard about a dysfunctional family in which the children feared their mother. They accused her in court of having subjected them to cruel punishments such as food deprivation. Porritt also claimed to have been physically and emotionally scarred as a teen after Mrs Porritt confiscated his prescriptions for acne medication.

The judge found that Porritt had been ''criminally negligent'' when he inflicted 57 stabs and cuts on his mother during what Porritt claimed had been a struggle she had initiated in the family's Chapman home on December 21, 2005, after he showed up uninvited and unannounced despite not having spoken to his parents for almost a year.

Porritt spent 22 months in custody after being arrested at home in October 2006 and charged with murder.

He had been living with his father and sisters since being released on bail in July that year, after Queensland police arrested him on the Gold Coast. He had fled there to avoid a directions hearing for an inquest into his mother's death.

In March last year Porritt, under strict supervision, showed police where he had buried some personal belongings he had taken from his mother, including jewellery and car keys, and where he had disposed of the knife in the Acton waters of Lake Burley Griffin.

Porritt's defence at the trial was that he had not intended to kill his mother, and had not realised he was inflicting serious injuries with the knife he claimed he had prised from her hand during a struggle in her marital bedroom.

Chief Justice Higgins, who accepted Porritt's version of events, said yesterday he found a number of elements had amounted to criminal negligence.

''The gravamen of the criminally negligent conduct I found the offender to have engaged in was, firstly, that he refused to leave his parents' home when requested by his mother to do so ... [to] compound his fault, the offender not only remained in the house but, knowing his mother was going upstairs to call police to have him removed, he followed her. His decision to do so led to the fatal consequences.''

In Porritt's favour was that he was a bright young man who had no previous criminal charges and who had been successful in his career with the Australian Taxation Office before his mother's death.

Mrs Johnston said she was disappointed she and her husband had not been able to attend court yesterday, but they had been given too short notice for them to arrange any flights.