Convicted sex offenders are being paroled to the Auckland CBD where they have been caught associating with prostitutes as young as 13.

Police officers and outreach workers trying to stop underage prostitution say they are concerned about the number of sex offenders being released to live in downtown Auckland close to notorious red-light districts.

Senior Constable Mark Riddell, of the Auckland youth action team, said they had discovered a young girl in the apartment of a sex offender.

He said: "We couldn't charge him with having sex with her, but just by associating with her he breached his parole conditions so he is back inside."

Riddell was aware of other cases but could not reveal more details as they were ongoing investigations.

Police know of at least 12 paroled sex offenders who are living in the CBD.

And concerns have been raised that, in the midst of the problem, specialist officers from the youth action team were temporarily reassigned to deal with a spate of car crime.

Underage prostitution has gained national media coverage after it emerged that more than a dozen girls under 16 were selling themselves in the Auckland CBD. Child, Youth and Family Northern regional director Marion Heeney said the agency had set up a task force to deal with the "sudden increase" in under-age prostitution on central Auckland streets.

Debbie Baker, the manager of Streetreach, a group supporting street sex workers, said she was told police were "unavailable" after spotting underage girls in a red-light district.

"About three weeks ago when we were out we saw seven underage girls and the police weren't available."

Baker said she was concerned the effort to stop underage prostitution was declining. "It's very much a concern to us that the guys we have been working with in the youth action team are not able to deal with this."

However, Inspector Andrew Coster said the youth action officers had not been removed from their "core business" of assisting at-risk girls.

The Auckland area commander said: "They were doing some work on car issues as part of their day-to-day work.

"They provided assistance and they were still doing their normal stuff and were available to deal with any critical issues on the youth side of things."

Coster said the problem had decreased because of efforts by police and CYF.

Community Probation Services regional manager Astrid Kalders said they tried to find accommodation for sex offenders according to their risk of reoffending, and where they could receive support from family.

"However, there are instances where accommodation options for some offenders are limited," she said.