Police believe thousands of convicted paedophiles are living in communities throughout the country, many of whom are going undetected as they fall off the radar.
But a new initiative in two police districts means the worst offenders are now being tracked with regular monitoring, which has resulted in several being sent back to prison.
The Counties Manukau child exploitation unit launched Operation Guardian last year to monitor convicted paedophiles released into their community. It is the district's equivalent of a child sex offenders register, and ranks offenders as high, medium or low risk.
About 2,500 convicted child sex offenders are being tracked in South Auckland, with a special emphasis on the worst 100 or so.
"We've got some bad bastards here in New Zealand, guys who have done some really bad stuff, guys who are renowned worldwide," said Detective George Grove, of the Counties Manukau child exploitation team.
"And some of these guys are living next door to schools and things, and that's really concerning."
Police looking for information on child sex offenders in South Auckland early last year noticed a dearth of readily available data, plus a lack of information-sharing between government agencies, Mr Grove said.
That prompted the operation, which includes regularly engaging with known paedophiles for psychological assessments.
"High-risk offenders were being released from prison," Mr Grove said. "They were still high-risk but there was no handover. The number of child sex offenders who get deported back here, that was huge as well, and we didn't know about them because there was no sharing between the agencies."
Nine monitored offenders were recalled to jail last year after they ceased engaging with police during visits or avoided scheduled meetings.
"The really bad guys, we're looking at about 50. That's who we're focusing on. We're visiting them regularly; that's maybe fortnightly or monthly.
"About 40 medium-risk ones we visit every three to six months. We know where they live and who with and what vehicles they're driving. We're guarding the community, and they know that."
As part of Operation Haven, the Wellington child protection unit has identified about 2000 convicted paedophiles in the district, about 300 of whom are getting special attention.
The vast majority of child sex offenders in both districts are considered low-risk - but for those deemed high or medium risk, the screws are being tightened.
"There's big numbers," Wellington child protection boss Detective Sergeant Neil Holden said. "There's quite a bit of work, but clearly it's necessary for the type of individuals who are out in the community."
Detective Senior Sergeant Fleur de Bes, acting national manager for sexual violence and child protection, said the numbers in the two districts were probably comparable throughout the country.
The other 11 districts were being encouraged to follow the models under way in Counties Manukau and Wellington or put forward their own, Ms de Bes said.
The two operations are in line with the Government's plan to develop a national sex offenders register, to be shared between interested agencies.
Anne Tolley, minister for both police and Corrections, said work was well advanced on the register.
"Nationally, police and Corrections started working more closely together last year to share information and intelligence in what was one of the first steps towards a sex offender register."
Roll of dishonour
• About 2,500 convicted child sex offenders are living in Counties Manukau police district.
• About 2,000 convicted child sex offenders are living in Wellington police district.
• The vast majority of paedophiles identified in Operations Guardian and Haven are considered low-risk by police.
• Between about 100 and 300 in each district are considered high- or medium-risk and are being actively monitored and regularly visited and assessed.