The hunt has begun for who leaked the details of the sealed settlement former Premier Campbell Newman and former Attorney-General Jarrod Bleijie made with a Gold Coast law firm, with the state's corruption watchdog confirming it has launched an investigation.

Last week, The Guardian reported Mr Newman and Mr Bleijie settled a defamation case with lawyers Chris and Daniel Hannay for more than $500,000, to be paid for by taxpayers under ministerial indemnity guidelines.

Campbell Newman had called lawyers who defended bikie gang members "hired guns" who were "part of the machine, part of the criminal gang machine". Credit:Renee Melides

It reportedly could have been lower, if a public apology was issued. Other sources have told Fairfax Media an apology had not been sought.