Deporting immigrants who breached their visa conditions cost New Zealand taxpayers $1.7 million in the past year.
Of the 671 people deported, 51 were sponsored migrants. Just $100,000 of the cost had been recovered from immigrants and sponsors by Immigration New Zealand to date.
Under the Immigration Act, a sponsor of temporary migrants will be liable for accommodation, maintenance and repatriation costs, as well as all debt owing to any government agency, and the sponsorship cannot be withdrawn.
A sponsor, who reported his international student girlfriend to Immigration after finding out she was working as a prostitute, said the legislation was "unfair".
"It's just totally disgusting and unfair that Immigration is penalising sponsors who do the right thing by reporting people breaking their visa conditions," said the sponsor, who did not want to be named.
The 56-year-old New Zealand man said he agreed to be a sponsor for the 21-year-old Chinese woman he met at a local cafe after she told him she could not prove to Immigration NZ that she had enough funds to remain in the country.
They later became a couple and had planned to marry after she finished her business diploma course.
But he felt something wasn't right when the girlfriend kept receiving calls from strangers.
Checking her mobile number online, he found out it was linked to an advertisement that offered sexual services.
"I confronted her, she admitted working part time in the sex industry and just walked out," he said.
"How is it fair that I am still responsible for her, especially after reporting that she has broken immigration laws?"
Immigration NZ area manager Michael Carley said sponsorship provisions had been strengthened under the Immigration Act 2009 to "better protect sponsored individuals and New Zealand taxpayers".
"When someone agrees to be a sponsor, they are responsible for the sponsored person from the day they arrive in New Zealand until the day they get a new visa ... or the day they leave, whether they are on a valid visa or in New Zealand unlawfully," he said.
"There is no provision to withdraw sponsorship and people need to make sure they know all relevant details about an individual's background and circumstances before they agree to become a sponsor in the first place."
In the last year, 2419 visas were issued to international students with sponsors linked to their applications.
Mr Carley said sponsors' responsibilities did not extend to ensuring the sponsored migrant did not break their visa conditions but they should ensure that people they sponsored did not undertake activities that would place them potentially liable for deportation.
"The obligations do not end if the sponsored person becomes unlawful. Any costs incurred until the person receives a new visa or leaves New Zealand are the responsibility of the sponsor."
The average cost for each deportation was $2200, and costs were often not recovered for some time after the deported person left, he said.
Mr Carley said the agency was case managing migrants facing deportation to undertake "voluntary departures" instead.