Shadow attorney-general Mark Dreyfus and opposition employment spokesman Brendan O'Connor wrote to former royal commissioner Dyson Heydon after the government offered a viewing of the confidential
volumes to crossbench senators.
The offer by Employment Minister Michaelia Cash is an attempt to win over crossbench senators and secure the six of eight votes needed to pass draft laws to reinstate the building industry watchdog.
The opposition and the Greens staunchly oppose the legislation, with its fate in the Senate resting with the crossbench.
Labor's letter notes a non-publication order was made for the secret volume in the interim unions report, however there was no such ruling in the final report.
It seeks advice on whether Mr Heydon intended for either volume to be released to the crossbench senators but not all parliamentarians.
"If so what was the justification for that decision?" the letter states.
The previously-rejected bills to reinstate the ABCC will again come before the House of Representatives on Thursday after a marathon day of debate on Wednesday.
The government has warned a second knock-back could lead to a double-dissolution election, something Prime Minister Malcolm Turnbull has said is a "live option".
Labor slammed the bills as "rotten to the core", arguing its previous operations were hostile and coercive.
Mr O'Connor said the "draconian" watchdog showed the government had an appetite to return to Work Choices.
"Here we are again, revisiting a bill that for more than two years hasn't had the support of the parliament," he said.
But the government maintains the Australian Building and Construction Commission must be re-established to stamp out the "poisonous" culture of unions in the sector.
Liberal Andrew Nikolic said since the ABCC's dismantling by Labor in 2012, the poisonous culture in the building industry had returned, especially by the construction union.
The CFMEU had repeatedly breached Australia's industrial relations laws and the government had a clear mandate to clean up the industry, he said.
"(These laws are) at the heart of ensuring this much-needed cultural change."
The opposition's letter to Mr Heydon follows one to Senator Cash on Tuesday accusing the government of engaging in the "most base transactional politics" and asking for justification.
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