National MP Mark Mitchell has accused Justice Minister Andrew Little of "trivialising" an indecent assault in which a prisoner grabbed the bottom of a female prison officer.

The case was raised by Little in Parliament on Thursday when Mitchell, National's justice spokesman, was questioning him about the numbers in prison who were "low-level criminals."

Mitchell had asked for an example of a "non-violent" assault after Little said more than 50 per cent of those who entered the prison system each year were convicted of crimes that did not involve violence.

Little referred to the conviction of the first "three-strikes" offender Raven Casey Campbell for indecent assault in 2016 for "pinching the bottom of a prison officer".

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He said that left Campbell facing the maximum seven-year prison sentence without parole because it was the Campbell's third conviction on a serious charge following convictions for robbery and aggravated robbery.

"The judge said he was not going to sentence anybody to seven years for pinching somebody's bottom."

Afterward Mitchell said the case in question was more serious than Little described it and he belittled the victim, who had to take stress leave as a result. The prison officer had said in her victim impact statement that she felt angry, frustrated and totally degraded and had since felt vulnerable and uneasy at work.

The judge's sentencing notes stated Campbell had "grabbed her right buttock, squeezed it quite hard and held on for 1 to 2 seconds." He had then refused to leave and had followed the guard to a gate and asked to talk to her.

"It beggars belief that the Justice Minister could so callously dismiss this as non-violent.

"He even attempted to minimise the offence by describing it as 'pinching' and suggested that it shouldn't have even been categorised as an indecent assault."

Little defended his decision to use that case as an example and said Mitchell was misrepresenting him. He said indecent assault covered a range of actions, some more serious than others.

"That action which that charge related to was pinching a prison officer's bottom. Now that is a world of difference from other actions that result in a charge of indecent assault that are genuinely more offensive, are violent."

The judge allowed for parole, saying it was not the most serious assault of its type and it would be unfair to impose a non-parole sentence of seven years.

Little is opposed to the three-strikes legislation and had sought to repeal it as part of his broader reforms of parole, bail and sentencing laws. However, that was dropped this week after NZ First said it would not support it.