Cabinet to decide on union investigation

Federal cabinet is expected on Monday to sign off on the details of a royal commission into the unions, with Prime Minister Tony Abbott saying it is necessary to "shine a great big spotlight" on corruption.

Mr Abbott says a wide-ranging inquiry is needed following repeated claims of union corruption, as Fairfax reported fresh allegations of kickbacks, bribes and organised crime links in the troubled construction industry.

Ahead of the return of parliament on Tuesday, cabinet will decide on the terms of reference for the royal commission, which will focus on bribes, secret commissions, kickbacks and slush funds.

"There've been very credible allegations, including allegations from senior officials in the union movement, of widespread corruption, standover tactics, even organised crime penetration," Mr Abbott told Sky News on Sunday night.

"And, as we have seen with other serious problems, serious criminal law-enforcement problems, sometimes you need to shine a big spotlight, a great big spotlight, into the dark corners of our national life."

Labor and the unions say the government should instead let police investigate the corruption claims, saying a royal commission is nothing but an expensive witch hunt.

Former High Court justice John Dyson Heydon is expected to head the 12-month inquiry, which will have the power to compel union leaders to give evidence.

Cabinet meets as Fairfax reports fresh claims of kickbacks paid to union officials in the Construction Forestry Mining and Energy Union (CFMEU) and plumbers union, as well as company managers, in return for contracts.