The government has put the hospitality industry on notice after a dozen backpackers were caught putting travellers to work as volunteers.
The Labour Inspectorate has been investigating 14 backpackers and has issued infringement notices to 12 for employing tourists in exchange for things like a bed, food, drink and wi-fi.
The volunteer arrangements breach immigration laws as none pay tax.
None of the wrongdoers have been fined, however, said the inspectorate's general manager, George Mason.
"We've put the industry on notice," he said.
"We expect them to clean up this problem - we'll be back auditing in six months and if we find places breaching the rules they can expect fines."
He said businesses in other industries may also be illegally employing volunteers.
"When you take a closer look at some of these schemes where businesses have workers putting in as many as 32 hours per week cleaning linen, working reception, and vacuuming, many of them are blatantly employment relationships and they are taking advantage of these workers," he said.
"Wherever a worker is being rewarded in a business at whatever level, the Labour Inspectorate's starting position is that these people are employees and minimum employment standards apply."
He said one of the backpackers has been reported to the Employment Relations Authority and could face further action.