A "sweetheart" visa deal between the federal government and Crown was axed following the arrest of Australian casino employees in China three years ago, with the gaming giant now bracing for a sweeping investigation that could examine its relationship with senior politicians.

Crown has promised to cooperate with the integrity watchdog probe but hit back at days of serious allegations about its domestic and international operations on Tuesday, labelling them a "smear".

Attorney-General Christian Porter said allegations of a cosy relationship between Crown and government agencies, including favourable visa processing for Chinese high-rollers, would be examined by the Australian Commission for Law Enforcement Integrity.

He said it was important there was "public confidence" in the nation's law enforcement agencies, including immigration and customs, and "all allegations of illegal activity" were taken seriously.