Opposition leader Bill Shorten has promised to change the law to protect workers' take-home pay if the Fair Work Commission cuts penalty rates in retail and hospitality sectors.

The announcement comes as the workplace tribunal has again delayed its decision on the issue, with business labelling Labor's response "premature".

In a speech to be delivered tonight at the John Curtin Research Centre in Melbourne, Mr Shorten will say he will not seek to legislate penalty rates.

"But if necessary, we would consider changing the rules, which guide the exercise of the commission's discretion."

"If the commission were to cut penalty rates for hundreds of thousands of already low-paid workers without sufficient compensation - leaving people worse off, Labor would not accept this."

The announcement suggests Labor may seek to legislate so that workers are compensated for weekend penalty cuts through other means, such as by a higher weekly rate, so they are not worse off overall.

Mr Shorten told reporters in Brisbane Labor will seek to prevent an "across the board reduction in workers take home pay" by ensuring the parameters in which the umpire sets wages "do not allow a reduction in workers' conditions".

Asked if he would accept compensating workers for penalty rates cuts by upping their overall pay, Mr Shorten said that was "a reasonable proposition".

"What Labor won't ever accept is people just having their Sunday rates just cut and that's it."

Decision delayed

The Fair Work Commission has sat for 46 days of hearings and received more than 150 submissions on penalty rates cuts in the retail, hospitality and fast food industries.

It was expected to deliver its decision by mid-January but on Friday it invited parties to provide submissions on new employment data, including on earnings and part-time work, with a deadline of February 2.

Australian Chamber of Commerce and Industry chief executive James Pearson called on parties to wait for the commission's decision before talking about responses.

"It is premature to discuss legislation to respond to the commission's decision on penalty rates," he said. "We should all wait for the commission's decision and review its approach and reasoning."

Shorten 'all at sea'

Mr Shorten's announcement goes further than his promises before the July election, where he resisted union calls to protect penalty rates and instead promised a submission to the commission if Labor was elected.

The Australian Council of Trade Unions said Shorten's announcement was an "important step in the right direction".

"The attack on Sunday penalty rates in the retail and hospitality sector will be the first domino to fall if the Fair Work Commission decides that Sunday rates can be cut," president Ged Kearney said. "We expect similar attacks on all sectors would soon follow."

However, Employment Minister Michaelia Cash said Mr Shorten was "all at sea on the issue".

"Only last year, he said he'd accept the decision of the Fair Work Commission's review of penalty rates, which he put in place."

"Mr Shorten should explain why, if he does not support a change in penalty rates, did he institute a review of penalty rates in the first place."

The Greens employment spokesperson Adam Bandt, who backs legislating penalty rates, said Labor's approach was similar to the current law that allowed trading off penalty rates for "vague 'compensation' elsewhere".

"Instead of simply putting a floor under existing penalty rates as the Greens propose, Labor's latest sleight of hand could still allow penalty rates to be cut."