A rogue employer in Christchurch New Zealand has been forcing Filipino workers to work for free to rebuild the city after the 2011 earthquake, a union says.
The First Union says a subcontractor made 40 employees work for free on Saturdays before the main contractor found out.
"It's one of the terrible situations where someone takes advantage," union southern secretary Paul Watson told AAP.
The 40 were employed by a reasonably big company, but were managed by another agency. That agency had since been sacked, he said.
The union says another case involves 15 employees who say they have not been paid for six to 11 weeks' work.
Filipinos make up the biggest single group of migrant workers helping with Christchurch rebuild following the 2011 earthquake.
The unions were hearing about exploitation cases through the Filipino community who were already living in Christchurch.
Mr Watson said some are not aware of their rights under New Zealand employment and health and safety laws. They were also scared of losing their work visa if they complained and were sacked.
"It's not good for the Canterbury workforce, and it's not good for New Zealand itself."
Mr Watson said government officials were moving quickly when they became aware of exploitation but migrant workers needed to be made more aware of their rights.
Immigration Minister Michael Woodhouse says the numbers involved are small, but there is a risk they could grow.
The government was distributing information about workers' rights both here and in the Philippines and also working on a law change to make exploitation of migrant workers a criminal offence, he told Radio New Zealand.