Labour Minister Kate Wilkinson says the 90-day trial period for new staff will be a boost for people who simply need an employer to give them a chance in a new job.
From Sunday, firms that employ fewer than 20 people will be able to take on staff on a trial basis.
Kate Wilkinson says the law gives businesses the confidence to take on new staff, and people who have been out of the workforce for some time the chance to prove themselves.
Council of Trade Unions president Helen Kelly says the law removes the right to fair dismissal from hundreds of thousands of employees.
Ms Kelly says bringing the law in during a recession is a terrible idea as people have even less choice about their employment.
She says the organisation will campaign hard to have the law repealed.
