Child-sex offenders list could be extended to include other criminals if successful.

All sex offenders and certain high-risk violent criminals could be included on a register being set up to cover child-sex offenders.

The scope of the controversial register would be widened from just child-sex offenders if it could be shown to be working, Social Development Minister Anne Tolley said.

That would see thousands more people forced to provide information including address, workplace and photographs - and would cost much more than the current $146 million over 10 years.

Ms Tolley this month introduced legislation to track convicted child-sex offenders for life, 15 years or eight years, depending on the severity of their crimes. "One of the discussions that we had at Cabinet as this was developed was how far do you go?" she told the Herald.

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"But because it is a new concept for New Zealand, and because there are significant human rights issues ... and because of the uncertainty of the quantification of the long-term benefits, Cabinet wanted to take it one stage at a time, so we have started with child-sex offenders."

The register's effectiveness will be reviewed three years after it begins operation around July next year.

"I would very much hope that at the time of that first review they would look at how it has worked to try and quantify a bit better some of the preventative work that has happened. And then look to expand out to cover all sex offenders, and maybe some of those high-risk violent offenders," Ms Tolley said.

The child-sex offenders register, to be accessed by Police and Corrections, will cost around $146 million over 10 years.

Ms Tolley said the potential costs of widening offenders on it had not been scoped. While the IT and monitoring systems would already be in place, the extra cost would be significant, she said.

It is estimated that about 2740 child-sex offenders will be registered after 10 years.

Statistics NZ data shows that in the 12 months to June, 1869 people committed sexual assault and related offences.

Police estimate that, over 10 years, the register could prevent between four and 34 child-sex offence convictions.

When Ms Tolley visited Britain in 2012, officials advised her of the value of a register there that included those who committed sex offences against both children and adults.

But NZ police noted insufficient information or evidence to quantify anticipated benefits from the child-sex offender register.

Ms Tolley said she was confident the benefits could be shown, and also brushed off Attorney-General Chris Finlayson's view that it does not comply with the Bill of Rights Act.

"I am positive that it will be discussed through the select committee process. But I am on the side of the kids," Ms Tolley said.

Labour leader Andrew Little said the party would want to see strong evidence that the child-sex offenders register was working and preventing offences before any move was made to widen its use. "We would want a close examination of what exactly it is that Anne Tolley is proposing, and whether it meets the objective of the original register."

On the list
•A register with up-to-date information on child-sex offenders, such as their address and workplace, could be widened to include all sex offenders, and some high-risk violent criminals.
•The child-sex offenders register is likely to be in place by next July, and authorised staff from Police, Corrections and agencies such as the Social Development Ministry will have access.
•There will be no public access, but in certain cases, such as where there is a threat to a child, data may be released to a third party, for example a parent or teacher.