A workers union is supporting Labour's proposition for a fairer 90-day trial system.

Labour leader Andrew Little has revealed a government he leads would retain the law which allows businesses to sack workers without justification inside their first 90 days of employment.

But he said he'll add a fairness requirement that would make an employer give feedback to a worker during the trial.

CTU president Helen Kelly says they support the 90-day trial as long as there are grounds for dismissal, and this would effectively mean the need for a whole new law.

"We understand Labour is consulting on how to do that. We would expect that to be effectively replacing it with the old probationary law and we'll wait and see what they'll do."

"The ninety-day trial as its drafted allows unfair dismissal and we see hundreds of cases where people have got no idea why they've been dismissed."