According to The Australian Financial Review, among the leaked communications are emails between two of the construction group's top executives that detail a "disgusting history of incompetence and misbehaviour" as well as "rewards, special bonuses, travel rorts" as part of its overseas operations.

It is understood that other emails, which will feature in a police case against former Leighton figures, warned a $A24 million “facilitation payment” linked to a 2010 Iraq contract would “attract attention” from auditors, according to the AFR.

The emails reportedly state that top Iraqi oil ministry officials were tipped off that one of their colleagues had been named as taking $US27 million ($A30 million) from Leighton.

According to the AFR, Australian Federal Police probing the bribery allegations are finalising a brief of evidence to present to prosecutors.

The newspaper reported that federal agents recently received advice from the Commonwealth Director of Public Prosecutions to complete their brief detailing bribery in Iraq.

The freshly leaked emails and ongoing police investigation tie in with the Australian Securities and Investments Commission’s investigation of potential corrupt dealings at Leighton’s international operations, which began in April this year.

Earlier this year Fairfax Media reported that ASIC chief Greg Medcraft told a Senate estimates committee that the watchdog began its inquiry in order to probe Leighton Holdings and see if it was in breach of Australian foreign bribery laws and of the Corporations Act.

Medcraft reportedly explained that ASIC planned to use its coercive powers to quiz witnesses and suspects over allegations made in October 2013 that Leighton paid multi-million dollar kickbacks to win contracts overseas.

At the time, a Fairfax investigation reportedly discovered that former Leighton International boss David Savage had signed off on an alleged $A42 million kickback to win a $750 million oil pipeline contract in Iraq in 2010.

The revelation last year cost Savage his seat on the board of UK engineering group Keller.

Other shady deals and potential bribes were also uncovered as part of the investigation, all of which form some of the basis of the AFP’s ongoing investigation into the matter.