A Newshub investigation has revealed hundreds of suspected sex workers are being stopped at the border, as Immigration New Zealand cracks down.
The practice is illegal, but that isn't stopping them coming here from Asia and South America.
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More than four thousand legal sex workers operate in New Zealand at any one time.
On top of that, there are "illegal foreign" workers, but officials don't know how many.
Officials say the problem is getting worse.
"We're concerned that some of these unlawful migrants are being exploited," said Dave Campbell of Immigration NZ.
The law says travellers on temporary work visas cannot provide sexual services, and foreigners with resident visas can't operate or invest in a prostitution business.
Despite that, hundreds are still trying. Between July 2015 and May this year, 481 suspected or known foreign sex workers were refused entry to New Zealand.
Another 43 were stopped from boarding or offloaded from aircraft before heading here.
Immigration NZ says it's cracking down to protect vulnerable women, often coming from Hong Kong, China and Brazil.
"We don't want to allow a situation where people are potentially being trafficked into the sex industry in New Zealand," Campbell told Newshub.
The practice was made illegal, so New Zealand wouldn't become a haven for sex-trafficking.
On Wednesday, Newshub spoke with one prominent South Island brothel owner, who says she knows it happens - the worst of it in Auckland.
"Those girls are brought in for the purpose of working," the unnamed woman said. "They normally don't speak a huge amount of English, so they don't have a lot of rights when it comes to understanding what the processes are."
Newshub has obtained official advice given to Immigration Minister Iain Lees-Galloway, which explains how some brothel owners get away with it.
Despite that, some, including the Prostitutes Collective, say migrants should legally be able to work.
"We encounter situations where sex workers have been badly assaulted but feel they're unable to come forward, because they're in breach of the legislation," said Catherine Healy of the Prostitutes Collective.
Recent research suggests most illegal sex workers aren't being exploited, but Immigration NZ says it will continue to investigate the industry, and notify brothels of their obligations.
It says anyone forced to stay here or engage in sex work against their will should contact officials, so they can stop the exploitation.