An advocate for migrant workers says a bill before Parliament that aims to crack down on exploitation cannot come soon enough.
Under the Immigration Amendment Bill, employers who exploit workers could be jailed for up to seven years, fined up to $100,000 and possibly deported.
A spokesperson for the Union Network of Migrants, Dennis Maga, says the bill needs to be passed as soon as possible because migrants working on the Christchurch rebuild are being ill-treated.
"They sign an employment agreement in the Philippines for example, and once they arrive in New Zealand, it is being altered."
The union says it knows of a case involving Filipino workers whose local employer who is threatening to send them home if they don't work during the weekend without pay.
At present it is a criminal offence to exploit illegal migrant workers, and Immigration Minister Michael Woodhouse wants to widen the net to include legal workers.
Mr Woodhouse says workers are being taken advantage of in many ways, including being paid below the minimum wage, being forced to work extra hours without pay, or having their passports withheld.