A former child social worker found guilty of sexually abusing children in his care at an Auckland residential school during the 1980s has been sentenced to six years' imprisonment.

Graeme McCardle, 59, was found guilty in August of a total of 15 sexual and physical abuse charges, against a boy aged nine or 10 at the time, and a girl aged between 12 and 13 years old.

He was cleared of another nine charges. He denied all the charges.

McCardle provided the daily care for children at the Waimokoia Residential School, which has since closed, while they were not actually attending lessons.

Among the incidents McCardle was convicted for was one in which he forced the 12-year-old girl to perform sex acts on him in the school lounge while he watched pornographic movies.

He was also convicted of indecently assaulted a nine-year-old boy in the school's concrete, windowless time-out room.

"One of the mysteries of this case, Mr McCardle, is how a man who was otherwise well regarded by everybody around him, including many of those who worked at the school, could do the kinds of things that the complainants said that you did," Justice Graham Lang said in the High Court at Auckland today.

"I consider that the only rational explanation for this is that at this particular period in your life were undergoing some form of crisis in relation to your sexual aims and ambitions," Justice Lang said.

The Crown had asked for a starting point of between eight and 10 years' imprisonment. The maximum penalty in relation to charges of unlawful sexual connection is 14 years' imprisonment.

The children were particularly vulnerable at the time of the offending because they had been sent to the school as a result of trouble they had encountered in the outside world and within their own families, Justice Lang said.

They also had no-one to talk to about the abuse, which was carried out over a long time.

"Twelve months might not seem a long period to an adult, but to a child living in a residential school such as this where this type of offending was occurring reasonably frequently, it must have seemed an eternity," Justice Lang said.

However, he gave McCardle credit for his otherwise blameless life and "real standing" in the community.

McCardle was promoted to security manager at Auckland International Airport last year.