The Council of Trade Unions is concerned proposed changes to the minimum wage calculations will affect health and safety.
Currently, wages can be calculated hourly, daily or weekly and the Government wants to add fortnightly to that list.
The CTU believes the proposal is a response to lobbying by the farming sector, which wants staff to be able to work long hours one week and fewer the next, without having to pay overtime.
CTU general counsel Jeff Sissons says longer hours will make workers fatigued and that could lead to accidents.
He says last year alone, there were 18 agriculture-related deaths.