Prime Minister Tony Abbott is expected to announce a royal commission into unions and their finances as Federal Parliament prepares to return this week.

The judicial inquiry was promised by the Coalition in opposition, as part of the fallout from the political brawl over the AWU slush fund former prime minister Julia Gillard says she unknowingly helped establish for her boyfriend during her time as a lawyer at Slater and Gordon.

Mr Abbott has been urged to widen the scope of any inquiry to look more broadly at union corruption following new allegations uncovered by the ABC and Fairfax of systemic corruption within the construction industry.

However the royal commission, to be announced this week, is expected to narrow the focus on how unions use their members' fees.

Attorney-General George Brandis says there has been growing public concern about corruption within trade unions.

"I think it would be, frankly, irresponsible for the Government not to respond to that growing demand in a very thorough way," Senator Brandis told Sky News.

It has been reported that the royal commission would be led by retired High Court justice Dyson Heydon and would carry out its investigations over a 12-month period.

Labor is opposing the royal commission, which it says is politically motivated, and favours a police taskforce instead.

Opposition Leader Bill Shorten says no-one is above the law and that criminal issues are a matter for the police.

He has also attacked the cost of the mooted royal commission.

"The idea that we need to have an expensive multi-million-dollar political royal commission, rather than give those scarce resources to our hardworking police, means the wrong priorities," Mr Shorten told the ABC.

Instead, Mr Shorten says Labor is calling upon the Government to set up a federal police-led taskforce to deal with the most serious allegations of corruption and bribery.

Senator Brandis says he expects Mr Shorten to protect union bosses because he used to be one.

The Government is this week expected to bring on a vote in the Lower House on its legislation to re-establish the Australian Building Construction Commission, which polices the construction industry.

The legislation is expected to be blocked in the Senate because Labor and the Greens both oppose the ABCC, believing the move to re-establish it to be an attack on the construction union.

Nothing but a political inquiry: ACTU

The ACTU has branded the move to establish a royal commission as an attempt to weaken unions and an attack on the take-home pay of working Australians.

ACTU president Ged Kearney also questioned the likely cost of the inquiry, coming days after the Government rejected a $25 million rescue package for Victoria-based food processor SPC Ardmona.

"This is $100 million of taxpayers' money being wasted, wasted on nothing but a political inquiry for Tony Abbott when just last week he could have put $25 million into saving 3,000 jobs and perhaps an entire rural community," Ms Kearney said.

"Australians want a government to stand up and fight to keep jobs in Australia like the jobs at SPC.

"They don't want a Government who is going to attack their take-home pay.

"The real issue unions are fighting for is to ensure that wages keep up with the cost of living, and the Government has spent the last few weeks attacking ordinary factory workers and their take-home pay.

"Make no mistake, the $100 million political inquiry is designed to weaken unions so that we can't stand up and fight for Australians' standard of living."

Ms Kearney says that while the Government was yet to release any detail on the terms of reference for a royal commission, any inquiry into allegations of union misconduct would be better dealt with by police.

"The ACTU has always said we have zero tolerance of corruption, and if there is corruption it should be pursued in the legal avenues via the police," she said.

"The police should be empowered. They should be helped and supported to pursue those allegations."