Maori Party co-leader Te Ururoa Flavell said the main source of his party's opposition was the "ultimate deterioration of collective bargaining".
"This Bill [Act] will certainly affect vulnerable workers who are going to be at a disadvantage as a result of the limitations to collective bargaining."
In the process of the bill passing, National argued having guaranteed tea breaks was impractical for certain work sectors.
Mr Flavell said he agreed the Act would offer more flexibility, but "only to a degree".
"While it will make the workplace more flexible, it is giving the employer the power to take advantage of vulnerable workers while stripping those workers of any bargaining power they had."
Rotorua Chamber of Commerce chief executive Darrin Walsh said they had heard a lot about the negatives, but agreed it would "bring back some flexibility to negotiate both ways and would bring fairness for both employer and employee".
"There are a number of changes to the Act and to be fair the more controversial one is that removes the right for tea breaks if it is deemed unnecessary due to the nature of the job.
"The removal of a break would be compensated by the employer, time in lieu for example, but obviously there would be negotiation involved and I wouldn't see breaks being removed from labour intensive roles where people need to have a break."
Mr Walsh said the changes were ultimately fair and "if you are a good, honest, hard-working employee you have nothing to worry about".