Ninety-day trial periods failed because employers that have tried to use them to hire-and-fire have often ended up having to pay large settlements, says the Unite Union.
A Treasury-funded study released on Friday found the controversial legislation has had no statistically significant impact on employment.
"My research shows that the 90-day trial period isn't helping people get jobs," said researcher Isabelle Sin.
"It doesn't make people less likely to leave secure jobs and doesn't make employment relationships less stable. Overall, my research suggests the 90-day trial isn't doing much at all."
Except put a lot of young, vulnerable workers through hell, says Unite organiser Joe Carolan.
"When National introduced the 90-day law, a lot of employers who backed it thought it would give them the power to just hire-and-fire without workers being able to speak back, and that that would baptise you in a workplace culture where you were docile, where you were worried that you could be fired for any reason at all," he tells Newshub.
"The experience of contesting that over the years has proven to us that there are other laws in New Zealand which do not end just because you are on day 88 of your employment."
This view is backed up by Dr Sin, who put it in more academic terms.
"The main effect of the policy was a decrease in dismissal costs for firms, while many employees faced increased uncertainty about their job security for three months after being hired."
At the time the law was expanded from small businesses to everyone, Labour Minister Kate Wilkinson said it gave employers "the confidence to hire new employees", and that without it, "hundreds of New Zealand workers would not have the jobs they currently do have".
But Michael Woodhouse, the present Workplace Relations Minister, denies job growth was ever the intention.
"The 90-day trial period was part of a basket of flexibility measures we put in to ensure employers had every tool they could to recruit when there was uncertainty, or to maintain when there was uncertainty, so frankly the report asks the wrong question, so is of limited or no value," he said on Friday.
"Has there been material job growth? That was never the intention."
Unite's most recent 90-day win came on Friday, a confidential settlement paid to an unfairly dismissed worker.
"It doesn't trump other laws that are there to protect workers, to protect them from discrimination, to protect them from undue or unfair process. This law has been just a paper tiger that we've been able to beat in practise."
Labour has proposed replacing the 90-day trial period to give more protections to workers, but Mr Carolan says many bosses know better than to even try.
"Most of your Queen St lawyers who give employment advice to bosses would back this up -- use this law at your peril."
Mr Carolan says those most at risk of being exploited under the 90-day law are those who aren't part of a union which can help them out.
"In nearly all of the cases that we've taken to mediation we've found that proper procedures weren't followed, due process wasn't followed, the reasons that the employer had for doing what they had didn't stand up to independent mediation, and in all those cases a substantial amount of money has been pain in compensation to the workers," he says.
"It's worth fighting."