The ACTU said the current rules do not match the reality of modern life, and working mothers and fathers - or employees with elderly parents - are struggling.
It wants workers to be entitled to temporarily reduced hours, and to have the option of returning to previous hours once caring responsibilities are finished.
Moreover, it has applied to the Fair Work Commission
for a new entitlement for workers to better manage their caring responsibilities.
The current rules mean that workers can ask for a change in hours, and employers can agree or disagree.
However, under the new arrangement, employers would be required to show cause if they didn't allow a request linked to unpaid caring.
The push comes after a survey of more than 5400 Australians found 85% have significant family caring or parenting responsibilities.
The survey, conducted for the ACTU, found 40% of workers have asked their employer for reduced hours for caring, but almost a quarter of these had been denied.
It also found that almost one in two workers needs access to reduced hours for caring, and women are almost twice as likely to ask for reduced hours for caring.
Moreover, employers were found to be 50% more likely to reject a male worker's request for reduced hours, and an inflexible workplace culture is the reason most cited for workers not asking for reduced hours to care for a member of family.
"We think people should really have the right to be able to reduce their hours on a temporary basis so they can actually fulfil their caring responsibilities," said ACTU president Ged Kearney on ABC
"At the moment you can ask your employer but your employer can simply say no and you have no right to appeal that."
The ACTU is launching a new campaign titled Change the Rules for Working Women & Families.
The case will be heard before a full bench of the FWC in December.
In July this year, it was announced that under a new FWC ruling, casual workers had won the right to request permanent roles after 12 months if they work regular hours over a year.
The FWC determined that a change to permanent work could occur if a “casual employee (over a calendar period of 12 months) has worked a pattern of hours on an ongoing basis which, without significant adjustment, could continue to be performed in accordance with the full-time or part-time employment provisions of the relevant award”.
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Employees should be able to reduce their working hours on a temporary basis so they can better manage their caring responsibilities, according to the Australian Council of Trade Unions (ACTU).