The major plastics manufacturer Sistema has been called "New Zealand's very own sweatshop", by a union that says it forces hundreds of migrant workers to work 12-hour days.
However, Sistema said in a statement the union's claims were completely false and that it was using "bully tactics".
The E Tū union said the mostly Fijian and Indian workers worked for minimum wage and often suffered from fatigue.
Sistema, which is run from a factory in Auckland, make plastic containers, and its website says these are exported to more than 100 countries.
Mandheer Aulakh moved to New Zealand from India in 2014 and worked as a machine operator for Sistema until early this year.
He said he decided to leave and take work as a bus driver, as working 12-hour shifts standing up, either from 7am-7pm or vice versa, was causing him major fatigue problems.
"My wife always, always complained about me - 'let's go to dinner, let's go to the beach' in my time off - but I said to her 'leave me alone, I just want to rest'," he said.
"Every day I also took Panadol as I could never sleep after a shift and would feel pain and swelling in my hands and back pain."
The machine operator said working conditions in India were far superior.
"The machines here are so fast and on some night shifts they would need more production and would increase the speed of the machine.
"When I got residency in 2015, I immediately started looking for other work."
E Tū's industry coordinator, Anita Rosentreter, said the company was exploiting migrants.
"When you've got workers doing 60-hour weeks, five days per week, on minimum wage without any additional allowances, and if they don't like it then they're told to get another job, those are the conditions of New Zealand's very own sweatshop," she said.
The union said it had been trying to negotiate a collective agreement since February.
Harry Singh also left the company earlier this year after suffering from back injuries.
He worked for the company for more than three-and-a-half years, and said he often worked 72-hour weeks.
"People don't speak up much because they want work, you know it's very hard to find work in New Zealand," he said.
"A lot of people work there who are paying off a mortgage - I was paying off my mortgage too - that's why I stayed quiet."
He said the management was very strict and any feedback was ignored.
"They just gave contracts to people and didn't ask them if they were happy with it, and if not they're told they can leave."
Union using 'bullying tactics' - Sistema
Sistema said in a statement it was "disappointed and frustrated at the completely false claims made by the union", and accused it of using "bully tactics".
"Sistema have over 700 employees - 266 are on the legal minimum wage and 175 of those have been here less than two years. Over the last two years Sistema have invested considerable resource to upskill members of our workforce," it said.
"We have a great relationship with our employees, many of whom have been with us for over 10 years, and in fact we have a waiting list of people who want to work here."
Migrant workers taken advantage of - report
Earlier today, a new report revealed cases of employers taking advantage of new migrants, with limited employment options and limited understanding of employment rights, by overworking them.
In some cases, employers demanded they work for free, the report - by Catholic advocacy and aid agency Caritas Aotearoa - said.
"These circumstances are depicted as 'normal' by the employer, and then taken on as a 'necessary sacrifice' by some migrant workers," it said.