Simon Bridges says the latest changes to employment laws will result in higher unemployment, lower wages for workers and less money for health and education.
From Monday, there are new rules around rest and meal breaks, 90-day trials, union rights and collective bargaining.
"The [Employment Relations Amendment] Act restores many of the conditions that existed during the previous Labour-led Government, at a time when the economy enjoyed record-low unemployment and unprecedented economic growth," Workplace Relations Minister Iain Lees-Galloway said in December, when the changes were passed into law.
"The Coalition Government believes everyone deserves a fair day's pay for a fair day's work. This Act helps achieve that by bringing back protections for workers, especially vulnerable workers, and strengthening the role of collective bargaining."
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The National Party leader told The AM Show on Monday the sky won't fall in as a result of the changes, but he's vowing to reverse them anyway.
"Ultimately it's about the business' confidence to hire more people, to take on more. This is why... growth is coming down. What does that mean ultimately? It means lower wages, it means we can't have the living standards we want, it means less money for health, for education, for infrastructure."
He said the Government is more interested in "protecting and helping Labour's mates in the unions" than boosting workers' incomes.
"There's worse to come in so-called fair pay. This effectively means all businesses, one-by-one over time, will be forced into bargaining with unions," Bridges claimed. "Not only that, they'll have to go along with what the unions and the Government force them to do."
The Employers and Manufacturers Association, which represents bosses, isn't sure that's the case. CEO Brett O'Riley told The AM Show they support the expanded entitlements to breaks.
"We definitely support the view workers... need the opportunity to rest," he said.
But the strengthening of collective bargaining is a "move backwards in time", said O'Riley, and the restriction of the 90-day trials for new employees to companies with 19 or fewer employees would make it hard for big employers to attract staff, while small firms struggle to use trials effectively.
"The flexibility of people being able to try different roles will be taken out of those larger firms... Typically smaller companies don't have the resources - they don't have an HR function or manager - to make sure the 90-day trial goes well. In some ways it's defeating the purpose of actually giving people a chance."
Of more concern to EMA members, said O'Riley, was the cost of complying with the new rules, rather than the rules themselves.
He said it's unlikely unemployment, which dropped to 4.2 percent in the March quarter, would rise.
The minimum wage went up in April by $1.20 to $17.70 an hour.
The unemployment rate fell constantly in the Helen Clark years, dropping from almost 8 percent in 1999 to 3.3 percent in 2008, before rising as the global financial crisis began to bite. It's been trending downwards since peaking at 6.7 percent in 2012.