A Senate inquiry has been asked to investigate construction firm Leighton's involvement in the $16.8 billion WestConnex project after damning evidence about offshore bribery practices from a former executive.

Last week former Leighton Holdings executive Stephen Sasse gave evidence about corruption within the company's offshore arm, Leighton International, to the inquiry which is examining foreign bribery.

Ms Sasse said Leighton International's board failed to act on concerns raised by a whistleblower in 2008 and 2009 about corruption on the part of its overseas executives.

He also said corporate regulator ASIC had told him it intends to charge businessman Peter Gregg with false accounting offences over a $15 million payment while he was Leighton's finance chief. Mr Gregg has described the allegation as baseless.