Council of Trade Unions president Helen Kelly said the removal of the right to appeal against unfair dismissal during the first 90 days of employment in small businesses would affect workers in 97% of enterprises.
The CTU estimated about 108,000 workers were in their first 90 days of employment in such work places at any one time.
"Over 700,000 New Zealanders start a new job each year and many of those are in small firms. Every worker can be affected at some time in their working lives. To remove this right of appeal means that a worker can be dismissed for whatever reason no matter how unfair, and have no right of appeal" she said.
Business New Zealand chief executive Phil O'Reilly said it was a policy used by just about every other developed country in the world - many with longer trial periods - and was an accepted part of their industrial relations framework.
The Government's decision to progress the legislation under urgency was not unreasonable since there was already an extensive select committee process with more than 600 submissions considered.
"This policy does not remove any of the human rights protections for employees under law. Most important, we are facing an economic downturn when jobs are most at risk and small businesses are least likely to hire. It is a pro-worker policy."
The least skilled, most marginal employers, at present most at risk of not gaining jobs, would get the most benefit from it, he said.
Mr Key said the 90-day legislation was not part of any hidden agenda as suggested by Labour leader Phil Goff.
It had been included in National's election manifesto and written about in every newspaper in the country.
