In the past three years, 42 foreign nationals have been found illegally working in the sex industry by Immigration New Zealand.
Twenty-five came on visitors' visas, eight on student visas, seven on work visas and two were overstayers.
Immigration adviser Tuariki Delamere, who had represented more than 30 foreign sex workers on immigration matters, believed the actual number of illegal prostitutes was much higher.
"Immigration's figures are only of those detected, but my guess is the numbers are possibly 10 times higher," he said.
"The law is just stupid, it drives foreign prostitutes underground and makes it hard to ensure occupational health and safety laws are kept."
Under the Prostitution Reform Act 2003, only New Zealand citizens and residents can work in the sex industry.
Mr Delamere said because of the law, foreign prostitutes were not accessing local support agencies and did not know where to get free condoms that were available for sex workers.
"The big worry, of course, is that many of them start offering unprotected sex and put Kiwis at risk of sexually transmitted infections," he said.
New Zealand Prostitutes' Collective national co-ordinator Catherine Healy said it was not a new trend for prostitutes to travel to New Zealand to work.
"Some sex workers travel and work in other countries, just like people in other occupations," Ms Healy said.
"Regardless of their immigration status, it is very upsetting to see sex workers being prosecuted and condoms used as evidence against them.
"All sex workers must be able to work in safe conditions."
Ms Healy told Australian News website in May that New Zealand was the best place on earth to be a prostitute.
She said the laws around sex work were the world's most effective and credited the Prostitution Reform Act for the world-leading conditions.
Her comments were picked up by the New York Post and news outlets in Malaysia, the Netherlands and the United Kingdom.
Immigration spokeswoman Emma Murphy said robust processes put in place to screen people coming to New Zealand resulted in the increase in detecting non-bona fide visitors.
"Immigration actively works with other agencies, such as police, to obtain information on foreign nationals working unlawfully in the sex industry," she said.
"Our focus remains on addressing the drivers of illegal work by deterring those who seek to encourage and profit from it through deliberate non-compliance with immigration laws."
The Korean Consulate in Auckland said it would provide appropriate support for the Korean sex workers caught working here illegally if requested.
In October, three South Korean citizens, including one who passed a secret note to a client seeking help, were found to be working at a Union St central city apartment.
China nationals made up the highest number of illegal prostitutes found working unlawfully here, followed by Hong Kong and Taiwan.
