Leaked emails allegedly show senior Leighton executives discussed plans to inflate the cost of contracts to disguise kickbacks to corrupt officials.
The emails show senior staff within the construction giant looked at ways to hide a $24 million corrupt payment in Iraq by artificially inflating the costs of providing security, accommodation and other services, Fairfax Media has reported.
Another email reportedly shows two top executives referring to a "disgusting history of incompetence and misbehaviour" and improper "rewards, special bonuses, travel rorts" in its overseas operations.
Fairfax also claimed the Australian Federal Police are finalising a brief of evidence against Leighton to present to prosecutors.
Leighton has previously said it was concerned by the corruption claims but has also accused media of "sensationalising" the allegations.
The scandal has seen at least three former Leighton executives quit their subsequent posts with different construction firms.
Meanwhile, former chief executive Wal King has denied any knowledge of the alleged payments in Iraq.