The Victorian Government has announced an inquiry into penalty rates cuts, as thousands of workers rallied against the changes in Melbourne.

Last month, the Fair Work Commission announced Sunday and public holiday pay rates would be reduced for workers in the retail, hospitality, pharmacy and fast food industries.

The Federal Government argues the move will enable businesses to open longer on weekends, boosting employment.

But Labor has vowed to do everything it can to block the changes, and unions have strongly opposed the cuts.

Victorian Premier Daniel Andrews said a select committee would explore how the state can protect workers.

He said the Government would use the findings to lobby the Prime Minister to block the cuts.

"We will not sit by and allow the Federal Government to treat those who are the most vulnerable, the lowest paid workers in our community in this way," he said.

"This parliamentary inquiry will allow us to gather together so many sad stories, so many stories of cruel cutbacks, that will have a really big impact in families right across our state.

"We want to make sure that when we're lobbying, when we're debating and discussing, we've got the best and most accurate picture."

The report will be tabled by September.

Cuts 'will flow into economy'

Workers were also concerned about moves to restore the ABCC. ( AAP: Tracey Nearmy )

Shortly after the announcement, thousands of union workers rallied in Melbourne to protest against the penalty rate cuts.

The secretary of the Victorian Trades Hall Council, Luke Hilakari, said about 15,000 people took part in the march.

Former prime minister John Howard was heckled by protesters in Sydney. Another rally was held in Brisbane.

Protester Daniel Bradley said cuts to retail and hospitality would have a wider impact.

"When you cut pay for workers, there's less money going back into the economy," he said.

"Because now they can't go out, they can't spend their money and … that [has] a flow on effect."

Some Construction, Forestry, Mining and Energy Union (CFMEU) members at the protest said they were concerned penalty cuts would soon be made to other industries.

"We do long hours and most of those hours are made up of penalty rates, and we rely upon these rates to survive," one said.

Workers also spoke against the Federal Government's move to restore the Australian Building and Construction Commission (ABCC).