A union claims migrant workers in Canterbury are being kicked, hit, and physically assaulted by their employers.

First Union is investigating, after being alerted to several incidents in the dairy farming sector.

Spokesperson Dennis Maga said the union is looking into one case where the employer lashed out because the worker was unaware how to operate farm machinery.

"He struggled to follow the instruction of the employer, the employer actually took a plastic bottle with water against the person and was kicked several times."

Maga said the problem is more widespread than many people are aware of - and more often than not, workers don't know where to turn.

The Ministry of Business, Innovation and Employment's labour inspectorate isn't aware of any specific cases of physical abuse.

General manager George Mason said it's a police matter at the end of the day.

"It's not something that we do. We're concerned that our workers in the Canterbury rebuild are treated properly so if there's concerns about abuse or bullying or assault then we need to know about that."

The number of complaints to MBIE about cases of migrant exploitation is on the decline, with various initiatives implemented in recent months to help tackle the problem.

Mason claimed this shows the MBIE is on top of the problem.

"We won't rest until we can be sure that every worker is going to be well looked after."