Mr Shorten's comments came as reports suggested the commission could hand down its decision by July, potentially lobbing it into the middle of a knife-edge election campaign.

The concession would appear to undermine Labor's ability to use penalty rates as a political weapon, in which the Coalition is accused of plotting to cut penalty rates and Labor is cast as their protector. It also threatens to put Labor at odds with its key union backers, who have pledged to fight any adverse decision.

Fair Work is reviewing penalty rates for seven retail and hospitality awards, and will evaluate demands by employers to bring Sunday rates down to Saturday levels. Asked on Melbourne's 3AW whether a Labor government would accept the commission's decision, Mr Shorten said: "Yes."

"I've said I'll accept the independent tribunal," he said. While Sunday rates should not be cut "just like that", it was ultimately up to the commission, he said.

"I've got my opinion. At the end of the day though, the way minimum wages get set in this country is through evidence, it's through the submissions of workers, their representatives and employers," Mr Shorten said.

The Coalition seized on the remarks, with Employment Minister Michaelia Cash labelling Labor's campaign on penalty rates a "sham", and asserting the ALP's position was now the same as the Coalition's.

However, Labor made a submission to the commission defending the current system of penalty rates, while senior members of the Coalition have openly entertained the idea of cuts. Prime Minister Malcolm Turnbull has described the Sunday allowances as a quirk of history that would inevitably be done away with. Under Tony Abbott, the government referred the matter to the Productivity Commission, which in December recommended the FWC reduce Sunday penalty rates.

The Greens immediately sought to draw a distinction with Labor, promising to consider legislation that would get around any decision to cut penalty rates.

"The Greens will wait to see the commission's ruling, however we will not rule out legislating," employment spokesman Adam Bandt said.

As recently as last week, the country's biggest unions were calling on the government to guarantee penalty rates would go untouched. United Voice national secretary Jo-anne Schofield warned: "If the Prime Minister does not commit to retain weekend pay rates, workers will ramp up campaigning on this issue in marginal seats across the country."

On Thursday, Ms Schofield would not be drawn on Mr Shorten's comments but said the "critical issue" was that Labor openly supported penalty rates while the Coalition did not.

Australian Council of Trade Unions secretary Dave Oliver said it was "difficult to overstate the importance of penalty rates" and that any cut would be "a crippling blow" for hundreds of thousands of workers. The ACTU would "explore all its options" if the commission decided to cut penalty rates, he said.

Polls show penalty rates are overwhelmingly popular with voters. Even in conservative Coalition seats, support for maintaining or increasing Sunday penalty rates exceeds 70 per cent, according to recent research.

Mr Shorten compared his non-intervention stance to the government's recent abolition of the independent Road Safety Remuneration Tribunal, which published a controversial ruling on minimum pay rates for owner-driver truckies. "When the government didn't like what the tribunal did, they abolished it," Mr Shorten said.

Labor sees industrial relations as one of its electoral strengths, particularly since the successful 2007 campaign against WorkChoices. With a July 2 election looming, the commission's decision may feed into a broader debate about workers' interests and union corruption.

The benchmark turnaround time for Fair Work to hand down its decision is three months from the date of the final hearing, which would mean about the middle of July. But it could come earlier or later, Fairfax Media understands.

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