Unions fear the ABCC makes it harder for union and workplace representatives to act on health and safety,claiming that construction site deaths hit a 10-year high under the Howard-era ABCC.

If it caused so much debate,why does the government want to bring it back?

The Coalition has focused its continuing attempts to beef up the regulator by pointing out the civil violations and alleged criminality of the powerful Construction,Forestry,Mining and Energy Union,which has a reputation for militancy and a propensity for industrial action.

CFMEU officials have faced sustained attack - during the federal government's recent royal commission and in multiple civil court cases - over allegations of underhanded or potentially illegal dealings,intimidation and abuse of industrial muscle.

Federal Employment Minister Michaelia Cash has called the legislation to re-establish the ABCC"absolutely vital"to combating what she says is rampant thuggery on building sites.

The government also points to increased levels of productivity as a reason to bring back the ABCC. Between 2005 and 2012,industrial action in construction fell by half. Union officials also had to tread more carefully and exercise greater restraint for fear of heavy fines.

What would actually change if the ABCC came back?

Labor did not remove the coercive powers when it set up the new agency,FWBC,and those powers are still often used. But a return to the ABCC would strip away safeguards restricting the regulator's use of coercive powers.

Currently,a presidential member of the Administrative Appeals Tribunal has to authorise the use of a coercive notice before it is executed,and must be satisfied that other avenues have failed. However,under the ABCC,notices are authorised at the commission's will and as a first resort.

Under a revived ABCC,the construction industry would also face much higher penalties for breaches of industrial law than those for the wider workforce. The CFMEU says the proposition that one industry should be singled out for tougher penalties contravenes the fundamental principle of equality before the law.

What happens now?

Because Labor and the Greens oppose the ABCC,the government needs the backing of six of eight crossbench Senators to get it over the line when the Senate is called back on April 18. If that fails,Mr Turnbull has guaranteed there will be an early election. And then it will be up to the voters.

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