The seven-year prison sentence handed to a man who pinched a prison guard's bottom is being slammed as barbaric, grotesque and unjust.

Photo: RNZ / Claire Eastham-Farrelly

Justice Kit Toogood was forced to hand 25-year-old Raven Casey Campbell a seven-year sentence under Police Minister Judith Collins' three strikes law, despite saying the offending warranted a one-year sentence at most.

The three-strikes law dictates that a judge must impose the maximum penalty if a person has been convicted three times for certain violent or sexual offences.

Campbell had already received one strike for his first set of offending, which included receiving stolen property, possessing a knife, robbery, and demanding to steal. He got a second-strike warning after committing aggravated robbery.

Pinching the female prison officer's bottom was his third-strike offence.

Justice Toogood said while it was very harsh, he had no option but to impose the maximum sentence.

Criminal Bar Association president Len Andersen said it was barbaric.

"It is fundamentally wrong that if an offender has committed an [offence] that deserves imprisonment for a year, that they should be sentenced to seven times that, simply because of an arbitrary law.

It is barbaric because the sentence is not in accordance with what is seen to be appropriate for the crime," he said.

Another senior lawyer, Robert Lithgow QC, described the sentence as preposterous and cruel.

Labour Party justice spokesperson Jacinda Ardern said the law removed the discretion of judges and should never have been passed.

"The judge has indicated that seven years was disproportionate," Ms Ardern said.

"I think it's quite extraordinary to have an experienced judge like Judge Toogood, who has actually given an indication of the sentence he would have given had it not been for the three strikes law requiring him to give the sentence that he did."

Offender got what he deserved - ACT

The three strikes law was driven through Parliament by Ms Collins and passed six years ago.

She assured the public at the time that appropriate safeguards were in place "particularly at the third stage of the regime".

Ms Collins said today she would not comment on a judicial decision.

However, the law was the brainchild of the Act Party. Current leader David Seymour said the offender got what he deserved.

"If you frame this penalty as only being for the most recent offence, yes, you will come to the conclusion that it sounds a bit crazy," he said.

"On the other hand, if you frame it as being the penalty for a series of offences and some quite serious ones - including aggravated robbery - then you put things in perspective."

Green Party justice spokesperson Metiria Turei said that while pinching the guard's bottom was an offensive thing to do, there was no way it justified seven years in prison.

"This is why we said at the time this law was stupid and shouldn't proceed - that it was always going to lead to inappropriate and ridiculous sentences that would massively increase the cost to the country of the prison population and be unjust," Mrs Turei said.

The architect of the three strikes legislation and former ACT MP David Garrett said people needed to realise the purpose of the law was to make the consequences exponentially worse for people who continued to reoffend.

Mr Garrett himself resigned from Parliament in 2010 after disclosing that he used a dead child's identity to obtain a false passport in 1984.

He was convicted in 2002 of assault in Tonga, and in 2012 he pleaded guilty to drink driving.