Immigration New Zealand is warning motel operators who let their rooms to foreign prostitutes that they could face huge fines and imprisonment and be charged with "aiding and abetting" illegal sex workers to remain in the country.

The department said it was expecting a surge in overseas sex workers coming into the country to meet an increase in demand during the Rugby World Cup, and will be closely monitoring the industry.

It is illegal for visitors on temporary visas, such as tourists and students, to work in the sex industry.

At least one brothel keeper said he was considering applying for a motel licence, and others thought they would get around the law by treating the prostitutes as "tenants" rather than "contractors or employees".

But an Immigration spokeswoman said as long as they received payment for the rooms, motel owners would have probably breached the Immigration Act.

"Given that the motel owner will receive payment, it is likely that they will contravene section 343(1)(a) of the Immigration Act 2009," she said. "Any person who allows a foreign national to provide commercial sexual services is aiding and abetting a person to either remain in New Zealand unlawfully or breach the conditions of their visa."

The act carries penalties of up to seven years' imprisonment and/or fines of up to $100,000.

The spokeswoman said motels would be searched if they were thought to be housing illegal sex workers.

"We are able to undertake compliance checks for breaches of immigration law at any premises," she said. "If we wish to serve a deportation order or execute deportation, we can enter and search any premises by force if necessary."

The pay-by-the-hour Desire Love Motel, which opened on the North Shore last month, is seen as a "working example" of how the owners of premises used primarily for sex are kept separate from the immigration status of people who use it.

"We provide a place for couples to meet, spice up their relationship and have some fun," owner Fred Ronnau said. "Discretion is part of the business, and it is mad for Immigration to suggest we check their passports.

"It's just totally absurd and against privacy law to be asking people who come here if they have a visa to be in New Zealand."

Fantasy rooms are available to couples in themes of Cleopatra, Kama Sutra, Savannah, Moulin Rouge, Disco and a large Roman room for threesome or groups.

A previous application by the motel for a brothel licence was declined after a public uproar.

The Motel Association said it would be against the law for moteliers to check someone's immigration status or not let rooms based on their ethnicity.

"Even if they suspected that someone is an illegal prostitute, what right have they to ask if she's got a valid working visa or not?" asked association chief executive Michael Baines.

"Of course a motelier has every right to ask her to leave if she was conducting commercial sexual activity there, but that applies to any prostitute, not just the foreign ones."