Two-year parental leave provisions in new ''family friendly'' workplace laws could unintentionally hurt women's careers, some academics and business groups have warned.

A new right for couples to request that unpaid parental leave be doubled, and for parents of young children to request flexible work arrangements, are ''legal changes that may lead to major social change'', University of Sydney professor of employment relations Marian Baird said.

ACTU president Sharan Burrow hailed the provisions, contained in the National Employment Standards that came into force on Friday as a ''crucial breakthrough'' for families.

But employer groups fear the changes will cause significant disruption to small businesses.

And Professor Baird said that while the new rights would increase work options for women once they returned from maternity leave, the changes could exacerbate the silent discrimination against women who break their careers to have children, even though it is illegal.