The Fair Work Commission on Thursday morning announced existing levels of Sunday penalty rates paid in retail, fast food, hospitality and pharmacy industries will be reduced from the existing levels, which, in some cases, are as much as "double time".

Full-time and part-time workers in retail will have their Sunday penalty rates dropped from 200 per cent to 150 per cent of their standard hourly rate, while casuals will go from 200 per cent to 175 per cent.

Hospitality employees will face a reduction in Sunday pay from 175 per cent to 150 per cent, while casual hospitality workers' pay will remain unchanged.

Fast-food employees' Sunday rates will go from 150 per cent to 125 per cent for full-time and part-time staff, and casuals will go from 200 per cent to 175 per cent.

The pay cuts take effect from July.

The controversial decision will anger Australia's union movement, which has invested heavily in a massive campaign to safeguard weekend penalty rates across the country.

Union leaders argue that Sunday loadings of up to 200 per cent were crucial compensation for low-paid employees who sacrifice weekends and work unsociable hours.

Employer groups have been pushing for years for a reduction to penalty rates on Sundays, which they say are too high, no longer reflect community standards and are forcing businesses to close their doors on weekends and public holidays.

They claim a higher wage for Sunday staff is no longer justified in a 24/7, seven-day-a-week economy, where workers see no difference in working Sundays compared to Saturdays.

The Fair Work Commission has spent almost two years weighing evidence from more than 140 witnesses and 6000 written submissions.

Pressure for a reduction to penalty rates has increased since Australia's Productivity Commission in 2015 recommended cutting Sunday rates in line with Saturdays.

Although Prime Minister Malcolm Turnbull has previously referred to Sunday penalty rates as an outdated concept, the federal government has left reform on the issue to the independent umpire, the Fair Work Commission.

Labor leader Bill Shorten has indicated he would act to protect workers' take-home pay in the event of any reduction. Earlier this week, Mr Shorten said his party would "never support a decision that sees workers worse off".

"With wages growth at record lows and underemployment at record highs, there could not be a worse time to cut penalty rates," he said.

"Millions of Australian families rely on penalty rates to put food on the table – Labor will fight for these people and their penalty rates."

Mr Shorten said Labor was "greatly concerned" that the ruling could set a precedent that could eventually cut weekend penalty rates for other occupations, too, such as nurses or paramedics.