The Fair Work Commission will deliver its ruling in Melbourne at 11am, which will affect levels of penalty rates currently paid in several industries nationally.

Unionists from the Victorian Trades Hall Council are planning to rally outside the commission in Melbourne's CBD to "defend penalty rates" ahead of the decision.

The trade union movement has been fiercely opposed to the prospect of any cuts, saying penalty rates provide critical and fair compensation for low-paid Australians who work unsociable hours.

Employer groups, meanwhile, argue penalty rates of up to 200 per cent on Sundays are too high, no longer reflect community standards and are forcing businesses to close their doors on weekends and holidays.

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 a sweeping inquiry by Australia's Productivity Commission in 2015 recommended cutting Sunday rates in line with Saturdays.

The Turnbull government says it will accept the commission's ruling on the future of penalty rates. However, if the Fair Work Commission decides against bringing Sunday penalty rates in line with Saturday rates, the Federal Government will likely come under pressure from business groups to intervene.

Labor leader Bill Shorten has indicated he would act to protect workers' take-home pay in the event of any reduction.

The Australian Greens have pledged to oppose any cuts to penalty rates.