The Engineering, Printing & Manufacturing Union claims new employment laws coming into force today, 1 April, undermine workers' rights.
A 90 day trial period for new employees is now extended to all businesses, where previously it was for those with fewer than 20 workers.
And under the new laws, unions will require consent from employers to gain access to workplaces.
The EPMU describes the changes as negative.
However, national secretary Andrew Little says the union is finding a growing number of employers who say they will not use the new laws.
Telecom, which has about 7000 employees, will not adopt the 90 day trial period.
Foodstuffs Wellington, which has 1500 people in administration and distribution, says the trial period allows the company to take on people where previously it might not have - and it is going to use it.
Employment law Professor Gordon Anderson of Victoria University says he does not foresee the laws having that much of an impact.