The cost of housing New Zealand's child sex offenders in the country's prisons is costing taxpayers more than $334,000 every day.

As of September 30, 2017, there were 1108 prisoners, out of a total prisoner population of 10,400, serving a sentence of imprisonment with the most serious of their offences being some kind of child sex offence, a request to the Department of Corrections under the Official Information Act has revealed.

The average cost of managing a prisoner convicted of sexual offending against children, usually in areas segregated from the mainstream prison population, worked out to $302 per day - the same as any regular prisoner.

KEVIN STENT/STUFF Corrections minister Kelvin Davis reckons prisons need to be a place where sex offenders are rehabilitated, as well as punished.

Annually, the bill for housing sex offenders in the country's 18 prisons - or "adult correctional facilities", as the department calls them - is $121.9 million.

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However corrections minister Kelvin Davis was keen to emphasise that while the prison population and the cost might be large, it did not necessarily mean the country was overrun with paedophiles.

CHRISTEL YARDLEY/STUFF The bill for housing sex offenders in the country's 18 prisons is $121.9 million per year.

While child sex offenders made up 10 per cent of the prison population, they were only a very small part of total offending.

"For example, in 2016 there were 411 people convicted of sex offences against children under 16 years out of 64,709 people that were convicted overall – which is less than one per cent," Davis said. "Child sex offenders typically receive long prison sentences and therefore make up a larger proportion of the prison population at any one time."

"As corrections minister I have a focus on making sure we do as much as possible to rehabilitate people while they are in prison, to stop reoffending and help keep our communities safe."

CHRISTEL YARDLEY/STUFF Child sex offenders usually receive long prison sentences and, therefore, make up a disproportionate proportion of the prison population at any one time.

The department had a specialised intensive treatment programmes for child sex offenders at two treatment units – Kia Marama at Rolleston Prison and Te Piriti at Auckland Prison. There was also a 12-week intervention programme for child sex offenders deemed at low risk of re-offending, and individualised treatment delivered by psychologists.

Corrections also managed child sex offenders in the community while they were serving sentences, orders or parole conditions. This included Extended Supervision Orders which involved ongoing monitoring after a sentence was completed. An offender's compliance with conditions was closely monitored by experienced probation staff.

The department's national commissioner Rachel Leota said no further breakdown of the costs of segregating child sex offenders from the bulk of the country's prison population was available.

CHRIS MCKEEN/STUFF Spring Hill Prison is another facility where a proportion of the prison population are kept in a segregated area, away from mainstream prisoners.

"The information dies not currently exist in a form that can be readily supplied to you, and would instead require initiation of a project to extract, analyse and present the data in the form requested."

Confusing the situation was the fact that not all prisoners convicted for child sex offences were segregated from the mainstream prisoner population and not all segregated prisoners were child sex offenders.

A prisoner could be placed on segregation if they presented some kind of security risk, or if their safety from others was deemed to be at risk from other prisoners.

Prisoners could request to be segregated if they had such concerns, Leota said.

"Around 25 per cent of the prison population are voluntarily segregated. The reasons ... vary, but can include the nature of their offence - for example, causing harm to children - or potential retaliation from gangs, their physical appearance, or other factors which may make them vulnerable."

Just how vulnerable were child sex offenders once they were behind bars? That, said Leota, was hard to tell. Since 2014 - during which there were a total 2482 child sex offenders imprisoned at some point - there had been 155 alleged assaults of those prisoners by other prisoners. Of those alleged assaults, eight had involved the victim being sexually violated or raped.

Category 2014/15 2015/16 2016/17

Assault - no injury 13 19 17

Assault 29 38 28

Assault - serious 1 1 1

Assault - sexual 2 1 5

However the accuracy of these figures could be open to question, Leota conceded.

"This information is generated from our incident reporting system and may not have been substantiated or may be an unfounded allegation."

Could regular prisoners break into the segregated area of a prison and attack sex offenders? Unlikely, Leota said.

"Extreme care is taken with prisoner movements to ensure that segregated and non-segregated prisoners do not have contact.

Given the secure nature of prison environments, this scenario is ... extremely unlikely to ever occur and we are not aware of any incident."

Just over half of the incarcerated child sex offenders were European.

Ethnicity Child sex offence

Maori 357

European 560

Pacific 146

Other 24

Not recorded 21

"Prisoners are not required to provide information to Corrections about their ethnicity or citizenship to facilitate their day-to-day management. When a prisoner volunteers this information, it is not checked for accuracy."

Stuff.co.nz had other questions for the department, to which Leota was not able to supply fulsome answers: What does segregation for child sex offenders look like? Do they have their own shower time? Do they eat different meals? Do they get a separate wing?

"Voluntarily segregated prisoners eat the same meals as prisoners who are not segregated. All prison meals are prepared by prisoners in prison kitchens, and meet Ministry of Health food and nutrition guidelines," she said.

"Segregated units have similar regimes to mainstream prisoners regarding their daily routine, access to work and programmes."