Prime Minister John Key says making it easier for smaller employers to sack workers will help boost employment.
Mr Key on Tuesday defended the National-led Government's decision to rush the legislation through before Christmas.
While the National Party campaigned on a policy of introducing a 90-day probation period for new employees working for small firms, it never said it would be part of its 100 days of action plan.
Under the change, small employers will be able to sack new workers within 90 days of hiring them without having to justify the dismissal. It has been included in urgency legislation before Parliament and is expected to be law by Christmas.
The fact the legislation will be rushed through without being referred to a select committee has angered the Council of Trade Unions and the Labour Party.
Labour leader Phil Goff says it is an ominous sign that National is prepared to take away people's employment rights without giving them the right to have their voices heard.
The CTU says it is outrageous that the Government intends pushing labour law changes through Parliament under urgency and plans to campaign against the change.
Union president Helen Kelly said the organisation was assured that the law change would not be part of the Government's first 100 days of action.
Workers' union Unite is promising to protest if the 90-day probation period becomes law. Matt McCarten, who heads Unite, told Waatea News it will take action against any employer who uses it.
"We want to put this Government on very clear warning ... This is a sneaky little trick and we will actually ignore the law. We will picket, we will take the fight to any employer who uses this to sack vulnerable workers."
Mr McCarten says at any given time 100,000 people are in their first three months of starting a job.
Safeguards in legislation, says Key
Mr Key rejected criticism that the approach is anti-democratic, saying there will be safeguards in the legislation and the issues have already been widely canvassed.
The Minister of Labour, Kate Wilkinson, said there is no need to go to a select committee because fellow National MP Wayne Mapp's Member's Bill had been through that process.
Ms Wilkinson said this legislation is a softer version of Dr Mapp's, with a number of safeguards, including that it will only apply to firms employing fewer than 20 employees.
The legislation is also subject to mediation, and has to be done with the employee's agreement. Protection against discrimination and harassment still apply.
Business New Zealand, which represents employers and businesses, says the Government is justified in bringing in the 90-day trial period under urgency legislation.
Chief executive Phil O'Reilly says small businesses have been saying this is their main issue for some time. He says though his organisation did not push for the change to be introduced under urgency, he believes it is warranted.