About 40% of employers who have taken on staff using the 90-day trial say they would not have been able to employ a new worker without it, a Department of Labour report has found.
The scheme was introduced in 2009 for businesses with fewer than 20 people and extended to all businesses in April 2011.
Under the scheme, a worker fired within 90 days of starting a job has no power to raise a personal grievance for unfair dismissal.
The department's report found 34% of business owners did not take up the opportunity to put new recruits on a 90-day trial, saying their systems were already robust enough to deal with situations in which staff did not meet expectations.
The survey showed 80% of business owners retained their staff once the probationary period was over.
Some employers in the Department of Labour report told researchers they had noticed a better work ethic among new workers who were on a trial.
But Council of Trade Unions secretary Peter Conway says creating fear among workers is wrong and unproductive.
"The fear of an unreasonable dismissal is not really going to be the sort of motivation that will contribute to a good employment relationship or ongoing improvements in productivity."
Employers and Manufacturers' Association chief executive Kim Campbell says there will always be some who misuse the scheme, but they are in the minority.
He says it is working well and it means business owners can take more of a risk with who they employ.
The report looked at the experience of employers but the department does not know many workers have got a job under the scheme.