The Council of Trade Unions believes extending paid parental leave would cost far less than the Government claims, producing its own figures showing a cost of about $160 million over three years.
Submissions on Labour MP Sue Moroney's bill, which would extend the leave from 14 weeks to six months, were heard by Parliament's Government administration select committee on Wednesday.
In a submission supporting the bill, CTU economist Bill Rosenberg said he estimates the cost of parents taking the full 26 weeks leave would be about $160 million over three years, with an ongoing cost of $80 million a year beyond that.
That cost is based on the assumption the workers will not be replaced by temporary staff during their leave.
However, Mr Rosenberg says if more temporary workers were taken on during parental leave periods, the cost would be reduced because of tax adjustments, including the effects of GST and Working for Families.
Even greater savings would be made if the person taking on the job came off a benefit.
"It's a way to create employment that is actually quite low-cost," Mr Rosenberg said.
Finance Minister Bill English is standing by his estimate that the bill would cost $450 million over three years, despite the much lower figures presented to the select committee.
The Government plans to kill the bill at its final reading using its financial veto, arguing that the cost is too high.
Meanwhile, Business NZ's Paul Mackay warned the select committee that extending parental leave could make employers less inclined to hire "a woman of child-bearing age".
He said businesses could face extra costs having to retrain staff once they returned to work as they may lose their "sharp edge" during the extra three months off, and Business NZ wants Parliament to look at the costs further.
NZN