An ex-Christchurch school teacher who groomed and had sex with an underage girl has today been jailed for nearly three years.

Daniel Bruce McMillan, 46, was working with youth offenders at Kingslea School over 2014 – 2016 when he preyed on the girl when she was aged 15.

"This was not an affair. This was not a relationship," Judge Jane Farish told McMillan as she sentenced him at Christchurch District Court this morning.

"This was a man taking advantage of a really young woman, a girl, for sexual pleasure."

The court heard that McMillan had been a school teacher for at least 10 years – and a talented musician who played in many bands.

McMillan knew the girl, who went to another school, and who trusted and confided in him, the court heard.

"Dan was someone I would confide in. He was a teacher and I trusted him," said the girl in her victim impact statement read out today by her devastated mother.

She was "young and naïve" and ended up having a "crush" on him.

"Dan really preyed on my issues and how vulnerable I was," she said.

There were two separate incidents, the court heard, including where he had sexual intercourse with the girl in a bathroom.

McMillan was found guilty by a jury last month on four charges – one of sexual grooming and three of sexual conduct with young person under 16.

He claimed she was older than 16 at the time.

McMillan also claimed that she "came on to him".

But Judge Farish said he should have known much better.

"You were the adult. She was the child... You knew exactly what you were doing," the judge said.

The victim said she felt "weak and used" after the sexual assaults. She said it feels like her childhood was taken away.

It's affected her mental health and she has tried to take her own life.

"To me, Dan is not a man," she said.

"He is a coward who preyed on me when I was young and vulnerable."

The victim's mother told McMillan in the dock: "You have destroyed our family over the last six years.

"You took away the light and happiness from our family."

Crown prosecutor April Mills raised the vulnerability of victim and the age disparity, along with the abuse of trust by a trusted friend and confidante.

Defence counsel Grant Tyrrell said McMillan acknowledges the "harrowing" victim impact statements.

"He has caused her harm and he acknowledges that and apologises for that," Tyrrell said.

"He knows he needs help."

But Judge Farish pointed to comments McMillan made to a pre-sentence writer he claimed he was the victim, "that she came on to him" and he succumbed to the attention in a weak moment.

The judge said that was not the actions of a responsible adult – especially someone who at the time was working with vulnerable young people.

He nodded in the dock when the judge talked about the grave breach of trust.

Judge Farish sentenced him to two years and eight months imprisonment.

He was also placed on the child sex offender register.