Key senator crossbenchers Jacqui Lambie and David Pocock are putting on a united front against the government trying to push the legislation through by Christmas.
The bill includes multi-employer bargaining and the ability to force businesses to cater for requests for more flexible work hours.
Senator Lambie says she's waiting for a guarantee the changes will push up wages, which is the government's main justification for the bill despite opposition from business.
"Because the only one that seems to be selling that point is (Workplace Relations Minister) Tony Burke and the unions," she told reporters on Monday.
Business groups have brokered an agreement with the government to amend the bill so that businesses and workforces aren't roped into industrial action against their will.
But they want the government to go further, as does Senator Lambie.
"If they don't want to multi-bargain, they shouldn't be made to, regardless of size," she said.
The independent Tasmanian senator said keeping the building and construction watchdog - which the government has committed to abolishing - was also a sticking point.
"Absolutely ridiculous," she said.
"Somebody needs to be policing (the industry). I thought it was working quite well, to be honest with you."
Senator Pocock says the secured amendments were a good start, but there's still work to be done.
He's proposed splitting the bill and solely voting on the non-controversial aspects, such as measures to improve gender pay equity, before Christmas.
The government has said it won't negotiate on the key aspects of the bill.
But it needs the vote of at least one crossbencher on top of the Greens for the legislation to pass the Senate.
Senator Lambie said it would be "a miracle" if the full suite of changes passed by Christmas.
Australian Council of Trade Unions president Michele O'Neil said workers couldn't wait any longer for a pay rise, with wages stagnant for a decade and real wages going backwards for more than year.
Ms O'Neil also said business groups were trying to stop wages from rising by opposing the multi-employer bargain components of the bill.
"Nobody should be fooled by the business lobby," she wrote on Twitter.
"They oppose bargaining changes because they don't want workers to get higher wages by bargaining together."
Parliament is scheduled to rise for the year on December 1.
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