State MPs have granted themselves immunity from investigation by the Corruption and Crime Commission to enjoy special status as the only public officers not subject to its extraordinary powers, according to the watchdog’s chief.

In a sensational speech to Curtin University this month, CCC Commissioner John McKechnie declared a Barnett government overhaul of its Act in 2015 effectively meant Parliament would deal with offending MPs in-house.

“The consequence is that the commission has jurisdiction to investigate allegations of serious misconduct in respect of all public officers except members of Parliament,” he said.

During the speech delivered during the caretaker period on March 7, Mr McKechnie, a senior Supreme Court judge and director of public prosecutions before taking the helm of the CCC in April 2015, said the CCC’s original remit included politicians.

Under its original model, if the CCC received a complaint about an MP it would refer it to the Speaker or President who would notify the relevant procedure and privileges committee, which would send it back to the CCC for investigation.

But under the 2015 structural reform aimed primarily at transferring low-level misconduct from the CCC to the Public Sector Commission, Parliament “reaffirmed exclusive jurisdiction over its own”.

He said MPs were protected from scrutiny unless Parliament made a referral to a privileges committee, which lacked the CCC’s full suite of powers, including covert operations.

Mr McKechnie said the same immunity may not apply to MPs who are also ministers, which the CCC retained jurisdiction over, though “it is often difficult to determine in what capacity a person is acting when they enter into a corrupt bargain or take a bribe”.

It all came down to whether an MP’s actions were covered by parliamentary privilege, which only Parliament could determine, and under article 9 of the Bill of Rights could not be questioned anywhere else.

Mr McKechnie cited a recent example in which the CCC, while investigating former treasurer Troy Buswell’s traffic crashes, uncovered false answers to the Legislative Council prepared by former government staffers Rachael Turnseck and Stephen Home.

Despite being unaware of the breach and handing contempt findings to the staff, Parliament took the CCC “to task” in November for straying outside its jurisdiction after a committee decided the answers had been covered by parliamentary privilege.

“I will adopt a prudent, some might say timorous course in future on any matter which could conceivably fall within parliamentary privilege,” Mr McKechnie said.