A former Wanganui Collegiate School music teacher has been sentenced to six years in prison for sexually abusing a student over four years.

Trevor Gibbs, 72, was found guilty of five charges of committing an indecent act on a boy and one charge of sodomy.

The offending took place between 1974 and 1978 when the victim was boarding at the school.

The sexual abuse allegedly happened in Gibbs' office — sometimes in the attic space above — and at the teacher's home.

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The victim read out a victim impact statement in which he described the shame he has carried with him his whole life.

"Throughout my school life, I lived a double existence. I've had to battle against feeling like a victim," he said.

The victim said he had to learn to re-trust people again after years of emotional manipulation.

In the statement, the victim said Gibbs had sent him a letter several years ago in which he wrote he had "followed his career".

The victim said this was a horrible, threatening thought.

Judge Richard McIlraith described the impact of Gibbs' offending as "profound and significant".

He said Gibbs had groomed the victim and taken advantage of his position of power as a teacher.

Lawyer Annabel Maxwell Scott represented Gibbs and asked the judge to consider reducing the sentence due to Gibbs' contribution to society through music and the arts.

She asked he be given credit for the joy he has given to others, visiting old people's homes and playing the organ.

Scott also asked the judge to consider reducing the sentence because of Gibbs' age and ill health and said he would find it very tough in prison.

The judge said most of the conditions Gibbs suffered from were not dissimilar to things most 72-year-olds would experience and said age didn't entitle him to a discount in sentence.

Gibbs had a number of support people in the public gallery, two of whom quietly wept after the sentence was read out.