The National party says the Government's planned changes to employment law, focusing on collective bargaining, will have a negative impact on businesses.
Workplace Relations Minister Ian Lees Galloway says giving unions a boost, together with an increase to the minimum wage, will help kiwi workers get their fair share.
But the opposition says increased unionisation won't stimulate growth.
"I think the main message from this legislation is that this is not really about workers' rights at all," said Scott Simpson, National spokesperson for Workplace Relations and safety.
"This is about strengthening union rights and many of us remember a time in years gone by when that occurred and the unions did have domination and we don't really want to go back to that," he said.