He was from an unsettled home and as a senior social worker, Liddell was assigned to his case.
But under the guise of help, he took the child home for two nights, telling his wife the boy couldn't go to his own house.
The first night when the boy was asleep, Liddell went into his room and sat beside him on the bed, taking the boy's hand and placing it on his crotch.
The boy panicked and tried to scream but Liddell covered his mouth tightly to the point he began to lose consciousness. He then held the boy down and forced him to perform oral sex.
The next day, Liddell took the boy to a wooded hill in a park and lay with him.
Again, the boy resisted but the man overpowered him and again forced him to perform oral sex.
Judge Lawrence Hinton, who sentenced Liddell for the new charges in December, said the breach of trust was "profound".
"You worked to manage the care of this boy not to take him home for your own sexual pleasure. That was the complete antithesis of what this injured boy in hospital was entitled to.
"He was vulnerable, he was not only young but he was incapacitated. Not only that, but he was the product of an unsettled family life and you simply abused your position.
"You planned it and you tried to keep him quiet following each incident. This was ugly and sinister on any view.
"Your victim suffered and will have suffered on any sensible view obvious harm."
Judge Hinton referred to the "tightly written" words of the Attorney General's Reference of 1998, which he said were as true today as they were when written.
"This is as its heart a denial of elementary values, as the judge said, and repugnant."
However, since being incarcerated for a second time in 2004 for offending after he was released two years prior, Judge Hinton accepted Liddell is making good strides in his rehabilitation.
The court heard how the sex offender had completed programmes, went to fortnightly relapse prevention meetings and had created a circle of support.
His lawyer, Kelly-Ann Stoikov, said Liddell took total responsibility for the offending.
The results of his dedicated participation in the programmes he has completed "show a man who is full aware if the consequences of his offending - a man concerned and mindful primarily for the wellbeing of his victim", she said.
"To be reminded through proper, and most necessary, prosecution of the man he was has been painful for him. Most acutely, your honour, since gaining that terrible insight into the man he once was."
From a starting point of four years in prison, Judge Hinton gave a "modest" discount for his genuine steps towards rehabilitation and 25 per cent for his early guilty pleas.
Liddell was sentenced to two years, eight months in prison to be served concurrently with his current sentence.
In previous Herald articles, Liddell was referred to as New Zealand's most dangerous sex offender. He's now amassed 20 convictions for sex offences.
The Attorney General's 1998 reference to the case
"It is a well-settled principle that the crimes of this kind call for a sentence which expresses society's emphatic denunciation of conduct which is a denial of elementary values. Such crimes are committed against a particularly vulnerable and helpless section of society who are in fact its most precious assets. Children who are victims of prolonged sexual abuse are robbed of much of the joy and innocence of childhood and may be badly affected for the rest of their lives in their own personalities and their relationship with others."
