Access to the secret report has become central to securing Senator Lambie's crucial vote to restore the ABCC, and she voted against the bill, which was defeated by one vote, last week.
When he released his 1800-page interim report in December last year, Mr Heydon said the 87-page third volume needed to remain secret to "protect the physical well-being of those witnesses [appearing at the commission] and their families. This is unfortunate, because the confidential volume reveals grave threats to the power and authority of the Australian state".
In his letter to Senator Lambie, Mr Abbott said there was ample publicly available evidence supporting the need to restore the ABCC, the Howard-era watchdog dismantled by Labor in government, and gave an insight into the contents of the third volume.
"I can advise that the confidential volume reports on threats of violence, and an act of violence, against witnesses called or likely to be called to give evidence before the Royal Commission," Mr Abbott wrote.
