Unions are vowing to continue the fight against an employment law change passed by Parliament last night.

The 90-day trial period for new employees has been extended to all businesses.

Previously it was limited to those with 20 or fewer employees and the change was voted through 64-56 against strong opposition from Labour and the Greens.

Council of Trade Unions president Helen Kelly says there would not be any let up in the campaign against the 90-day law, which the Labour Party calls "fire at will" legislation.

"We will continue to campaign against this government's attack on work rights and its continuing failure to take adequate action against unemployment."

Ms Kelly said the Government had chosen to listen only to business lobbies.

"There are 150,000 New Zealanders officially unemployed, yet the Government's response is to weaken everyone else's job security as though that was the root of the country's economic problems," she said.

"It seeks to weaken wage bargaining when our wages are falling further and further behind Australia's."

Maritime Union general secretary Joe Fleetwood said the law, and another changing holiday laws also passed last night, would making life harder and the Government could expect a ramped up campaign.

"As workers see their workmates sacked for stupid reasons or no reasons under the 90 day fire at will law, then disquiet and concern will turn to anger."

He said key issues for workers were low pay, unemployment and insecurity of jobs by contracting out and casualisation but the Government did not address those issues.

Labour Minister Kate Wilkinson said during the third reading debate on the bill the probation period had been a success.

"Rather than the sky falling in, as was hysterically proclaimed, employers of small and medium-sized businesses gained the confidence to hire new employees," she said.

"It is a fact that without the trial period, hundreds of New Zealand workers would not have the jobs they currently do have."

Ms Wilkinson said businesses did not want to face a personal grievance if they hired someone who turned out to be unsuitable.

"They simply chose not to hire anyone. The 90-day trial has changed that."

Labour's Trevor Mallard said extending the trial period to all businesses was "just a continuation of the National Party's attacks on the rights of wage and salary earners and their conditions."

He said the law was not helping the economy and had been brought in for political reasons.

"It weakens the processes around job security, extends the range of reasons for dismissal, restricts substantially the right to appeal, and restricts the right to reinstatement," he said.