A campaigner for migrant workers believes the immigration system leaves employees vulnerable to exploitation.
It follows the sentencing of a Bangladeshi couple who've been jailed for underpaying and overworking staff at thier confectionary store. Mohammed Atiqul Islam was sentenced to four years and five months in prison on Friday, and Nafisa Ahmed has been ordered to serve two years and six months behind bars.
They paid employees $6 an hour - only a fraction of the minimum wage - and confiscated their passports.
Union Network of Migrants co-ordinator Mandeep Singh Bela says the root causes need to be addressed.
"In many cases [the migrant] is attached to a particular employer and a particular location of work, so migrant workers feel bonded with that employer...
"If they were to change job, they not only lose the job but the ability to stay in New Zealand... the repercussions are massive."
One of the workers claimed he worked 14 hours a day, seven days a week. He said he was exhausted and was told that if he didn't like it, he could go back to Bangladesh.
Bela hopes the prison sentences act as a warning to others, and that other employees being mistreated are encouraged to speak out.
"For us, we think that it sends a really tough message to those employers out there who conduct such gross exploitation."
A spokesperson for Immigration Minister Iain Lees-Galloway said an "in-depth review on migrant exploitation is underway and the minister will receive it later this year".
"In the meantime, MBIE continues to take enforcement action to address the exploitation of migrant workers."