Immigration New Zealand denies slashing the number of compliance checks it makes on employers who hire migrant workers.
Labour Party immigration spokesperson Darien Fenton has released figures showing visits to employers in the horticulture and viticulture industries, nearly halved in the past three years.
The number fell from 484 in 2009 to 251 last year.
Ms Fenton says checks have been slashed, and migrant workers need to have their legal rights protected.
However, Immigration New Zealand general manager Peter Elms says the number of visits in 2009 was higher than usual because of an increased focus on educating employers.
He says checks in the past two years are consistent with previous years.
Mr Elms says employers who exploit migrant labour will not be tolerated.