Bill Shorten has vowed to restore the Sunday penalty rates of every worker affected by the July 1 cuts.
Hundreds of thousands of Australia's lowest paid workers in the hospital, retail and fast food industries will start to see their weekend penalty rates cut from next week, under a decision of the independent Fair Work Commission.
The opposition leader told an ACTU conference in Sydney on Tuesday a Labor government would change the law to protect the take-home pay of workers.
"I promise you this: a new Labor government will restore the Sunday penalty rates of every single worker affected by this cut," Mr Shorten said.
Mr Shorten contrasted the cut with the tax break to be given by the Turnbull government to the highest income earners from July 1, following the axing of the "deficit levy".
The government argues the penalty rates decision was made independently and the minimum wage is due to rise by 3.3 per cent on July 1.
