Labor is preparing to fire a fresh salvo in the political fight over penalty rates, ahead of a long-awaited decision from the Fair Work Commission on whether to cut Sunday penalties.

Opposition leader Bill Shorten will promise in a speech on Wednesday evening that if the commission decides to reduce Sunday overtime - a move that could affect nearly 800,000 people in the retail and hospital industry - Labor will move to change the law to protect workers' take-home pay.

Opposition Leader Bill Shorten says penalty rates 'are not a luxury'. Credit:Penny Stephens

Mr Shorten's promise stops short of the Greens' pledge, during the 2016 election, to put a floor under penalty rates but enshrines in law workers' overall take-home wage.

The pledge to legislate in a speech at the John Curtin Research Centre in Melbourne goes further than Labor has previously promised.