A group supporting migrant workers is warning that visa changes might not stop the exploitation of former foreign students.
From 26 November, the government is abolishing the two-year employer-assisted work visa that requires students to work for a particular business, and replacing it with open work visas.
Anu Kaloti, from the Migrant Workers Association, said the employer-assisted visa had been used to trap former students in under-paid jobs, and to persuade them to pay thousands of dollars in return for a visa.
"A majority of the cases where the migrant workers have been exploited has been where the worker's visa has been attached to that particular employer," she said.
Mrs Kaloti said removing the employer link would greatly reduce the scope for abuse, but she was worried employers would find new ways of exploiting migrant workers.
"Initially it should have a huge reduction. To say it will be completely eliminated - I want to hold off and see what happens in the very near future because I'm mindful the exploiting employers can sometimes find other ways to work around the system."
Several former students have told RNZ they paid employers tens of thousands of dollars so they could get an employer-assisted visa. In some cases there was no job at all, and in other cases the former students were under-paid and forced to work excessive hours.
Immigration New Zealand said people who were currently on the employer-assisted visa could apply to have the employer removed from the visa on 26 November.
Asked by RNZ about situations in which foreigners paid thousands of dollars for jobs, Immigration Minister Iain Lees-Galloway said people who were being exploited should alert Immigration New Zealand and seek a new work visa.
"We'd want them to bring that to the attention of Immigration New Zealand," he said.
"That's exactly the kind of exploitative behaviour we need to hear about so that we can stamp it out and we would also of course look at what options are available to people in those situations. One of those options could be to apply for a work visa."
Immigration New Zealand approved 10,749 employer-assisted work visas in the 12 months to the end of June, up from 9700 in the previous 12 months. It also approved 16,340 one-year open work visas for former students in the 2017-18 year.
The changes announced by the government this week gives all current foreign students a three-year open work visa upon graduation.
However, anybody gaining a study visa after 9 August would get a three-year open work visa only if they were studying at Bachelor's degree level or higher.
Students studying at lower levels would be eligible only for a one-year open visa, but that would extend to two years if they studied outside Auckland.