Public submissions on the Department of Labour-led action plan took exception to comments about the lack of evidence of people trafficking in New Zealand, noting that does not mean it is not happening.

Advocates and academics went so far as to say it was "implausible" to believe women and children were not being brought into the country to work as prostitutes, urging law enforcement to investigate the link with international organised crime syndicates.

Detective Inspector Scott Beard, Auckland district field crime manager, said there was no evidence to back that up, but noted: "Human trafficking is about making money. Organised crime is about making money."

With a boom in the number of brothels in Auckland suburbs - many unregistered - and police no longer able to raid brothels without search warrants, Mr Beard said trafficking could be going on undetected.

Trafficked women are often brought into a country under false pretences of working in another industry, then exploited sexually to repay debt.

Travel documents and passports are confiscated by traffickers to force compliance, as well as threats of physical violence, reprisals and public exposure.

Those reasons, as well as language barriers, meant potential victims would be reluctant to come forward and lay a complaint with police, said Mr Beard.

"If no one knows where these brothels are, how do we know there's not women who have had their passports removed and are forced to work in prostitution?"

Susan Coppedge, a US federal prosecutor who spent a year in New Zealand studying trafficking,

profiled three cases which occurred before the law changes in 2002 that would now be considered for trafficking prosecution.

The most high-profile one was the "pink sticker" campaign of 1999 where Thai women were held against their will in Auckland brothels.

The focus on the international Asian sex industry led the Human Rights Commission to use pink stickers to publicise a safe house and fast track repatriation with travel documents.

However, many of the victims returned to Thailand before their traffickers could be prosecuted.

By Jared Savage | Email Jared