The Government plans to give increased rights to employers in a move it says will make employment relationships fairer.
But the Council of Trade Unions (CTU) has denounced the Employment Relations Amendment Bill as an attack on the wages and conditions of workers and says it will take a complaint to the International Labour Organisation.
The bill was introduced to Parliament on Friday and includes policies that the National Party campaigned on in 2011.
If passed, the bill would remove the obligation on workers and employers to reach a settlement, allow for partial pay reductions in cases of partial strike action, and would no longer require workers to be covered by collective agreements for their first 30 days in a new job.
Labour Minister Simon Bridges said the bill would speed up the process at the Employment Relations Authority, increase confidence and make it easier to get jobs.
Mr Bridges said fairness is about giving employers some of the same rights enjoyed by workers and their unions.
"Fairness is a two-way street. I think that there are things here where we are effectively saying what's fair for one is fair for the other. Where unions have rights in certain instances, they're also rights that the employers should have around, for example, partial strike action."
But CTU president Helen Kelly believes the bill will remove workers' rights and make it easy for employers to simply walk away from collective agreements.
"What the Government is doing is allowing employers to say, 'I'm not going to bargain, because I don't want to a collective' - and that's a huge change.
"It's right back to the days where employers could promote individual employment agreements instead of having a duty to conclude a collective. It makes good faith bargaining a farce."
Helen Kelly said employers will be able to walk away from collective agreements under the bill.
"The other big thing it does (is) it removes the protection for new workers starting a job. At the moment, they have to be offered the collective for the first 30 days and then they can decide whether to join the union.
"What will happen now is that they'll be offered individual agreements that reduce their pay and conditions - and because of high unemployment, they'll be vulnerable to that and they'll have to accept it."
Ms Kelly said Simon Bridges hides the real effect of the changes by using pretty words like flexibility and choice - but the bill would even remove the tea break.