Labor wants answers on unions inquiry

Labor has written to the head of the royal commission into union corruption demanding to know what orders were made regarding the confidentiality of a volume of its report.

Shadow attorney-general Mark Dreyfus and employment spokesman Brendan O'Connor wrote to Dyson Heydon after the government offered a viewing of the secret volumes to crossbench senators.

The offer by Employment Minister Michaelia Cash is an attempt to win over the cross bench 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.

Labor's letter notes a non-publication order was made for the secret volume in the interim 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.

It urges a prompt response from Mr Heydon, given the government's intention to "imminently" release the confidential reports.

Labor and the Greens have rejected an offer for one delegate each to view the volume, with the opposition saying the confidential report had been handled incorrectly.

The opposition's letter follows one to Senator Cash on Tuesday, accusing the government of engaging in the "most base transactional politics" and asking for justification.

Legislation to reinstate the Australian Building and Construction Commission is being debated in the House of Representatives, where the government has the numbers to push it through.

The previously-rejected bills will then go to the Senate, where crossbenchers are expected to determine its fate.

The government has warned a second knockback could lead to a double-dissolution election, something Prime Minister Malcolm Turnbull has said is a "live option".