Opposition parties and trade unions are lining up to slam a controversial new employment law.

The Greens describe the Employment Relations Amendment Act as "mean-spirited and backward-looking legislation".

"This bill will force wages down, keep workers in insecure jobs, and is a blatant attempt to undermine unions," says the Greens' industrial relations spokewoman Denise Roche.

The new Act was passed this afternoon by 62 votes to 58.

It changes workers' entitlements to rest and meal breaks, allows employers to walk away from collective bargaining and removes protections for workers in small businesses when that business is sold.

The Prime Minister John Key insists it's about flexibility.

"I think for the vast overwhelming bulk of New Zealanders they'll find absolutely no change to their workplace and those that do will actually probably welcome that because they'll have the capacity to negotiate a little more freely with their employer," Mr Key said.

But unions say the new law is a "shameful" attack on workers' rights to have smoko breaks.

The President of the Council of Trade Unions, Helen Kelly, says: