A Department of Labour survey has found that about one in five workers employed under the 90-day trial period has been sacked.
The trial period allows a new worker to be fired within the first three months of employment, without the right to take a personal grievance.
Prime Minister John Key was due to announce on Sunday an extension to the scheme to companies with more than 20 workers, as part of a package of workplace law reforms.
The report says employers are generally happy with the trial period but some employees felt vulnerable to unfair treatment and dismissal.
In the survey, 132 employers had used the 90-day trial period to hire someone and 29 of them had sacked a worker within the 90 days.
The Department of Labour said 74% of those employed on a trial period had retained their employment once the trial period was over and a further 5% were still working within the trial period at the time of the survey.
Employers said workers had been sacked for poor performance or for a poor attitude to work but some employees complained they had no idea why they had lost their job.
Employers and Manufacturers Association Northern spokesperson David Lowe says the firing figure should be put into context, given half of all workplace problems and personal grievances happen during the first year of employment.
Council of Trade Unions president Helen Kelly says she has serious concerns about the scheme being extended, particularly if the number of workers being fired continues to be high.
Ms Kelly also said her organisation wanted details of the report's questions and methodology in advance of its release but was refused.
Key 'has numbers' to pass labour changes
Prime Minister John Key says he already has the numbers to pass labour law reforms.
"We have reached agreement with the ACT party to support us so we have the numbers we believe to pass into legislation our recommendations," he said.
Mr Key says he'd be very surprised if National's other support party, the Maori Party, were to support the changes. He says trade unions have nothing to fear from the move.
John Key will announce the package of workplace law reforms during his address to the National Party annual conference in Auckland.