When a mother found out the "monster" she had invited into her home had been molesting her son, it was the second worst day of her life - second only to the day her daughter died.

"Whilst [my son] wears the eternal damage of your actions, I too live with the mental trauma of knowing that I let a monster into our lives and it was my choice to let you into our home," the woman told her son's abuser in an emotional address at New Plymouth District Court.

"That will haunt me for the rest of my life and it has taken a toll on us that is immeasurable."

Today, Clifford Wiremu Nathan was jailed for the historical sexual offending against a young boy he once regularly babysat.

For much of the sentencing, the Taranaki man stood in the dock with his head in his hands, becoming tearful at times.

In 2002, Nathan, a former in-home mental health carer now in his 60s, was living in Wellington where he was studying for a nursing degree.

It was while living in the capital that he met the victim's mother and became trusted to mind her son.

Nathan bought the boy gifts and cultivated a relationship with him that would eventually lead to sexual abuse.

He molested the boy at least twice a week, every week, while the victim was aged between 8 and 9.

Now in his late 20s, the victim has in recent times revealed to authorities the abuse he endured.

Nathan admitted the offending and appeared in court on two representative charges of sexual violation by unlawful sexual connection and two representative charges of indecency with a boy under 12.

While the victim was not present at the hearing, his mother was and while giving her victim impact statement she said her daughter had died only two years before she met Nathan.

Clifford Wiremu Nathan was sentenced in New Plymouth District Court on Tuesday. Photo / Tara Shaskey

"You preyed on a struggling solo mother, one who was already in a dark place due to my daughter's death and you took advantage of my vulnerability in order to prey on my child," she said.

"The day I found out you'd molested my son was the second worst day of my life, only trumped by the day my daughter died."

Nathan had stolen her son's childhood and his mental health, the woman said.

While the victim was "gentle, kind, considerate, thoughtful and intelligent", he struggled to lead a "normal" life and had told his mother the only reason he was still alive was to not put her through the trauma of losing another child.

Judge Gregory Hikaka said the harm caused to the victim and his family was palpable and ongoing.

The victim's vulnerability, the breach of trust, and the scale of offending were all significant factors in the case, he said.

Judge Hikaka referenced a pre-sentence report that said Nathan wasn't sure why the offending had occurred but he thought it may have been due to the sexual and emotional abuse he experienced as a child.

He had also struggled with his sexual identity and abused alcohol and drugs.

Nathan had acknowledged his actions were wrong and showed a high level of genuine remorse, the report stated.

A cultural report said Nathan had been excluded from church due to his sexuality and he was raped at gunpoint while he was a sex worker in his 20s.

Judge Hikaka accepted there was a connection between Nathan's early life experiences and the offending.

But given the aggravating features, imprisonment must be imposed, the judge said.

"You preyed on this child."

Following credits for early pleas, remorse and personal circumstances, Nathan was jailed for four years and six months and was added to the child sex offender's register.

- Tara Shaskey, Open Justice