Mr Duncan said the commission should only investigate allegations of criminal conduct, including bribery, abuse of public office and "payola". But investigations about breaches of the parliamentary code of conduct or declarations of members interest should remain with the parliamentary commissioner for standards.

The standards commissioner should receive all complaints, and decide first whether they were simply vexatious or made only for political advantage or with insufficient evidence. Then, the commission would investigate any non-criminal allegations, referring possible criminal offences to the integrity commission.

That would ensure the commissioner for standards remained central and parliamentary immunities and powers remained intact, and it would introduce "proportionality" to the treatment of different complaints.

Mr Duncan was commenting in a submission to an ACT parliamentary inquiry which is working out the detail of an ACT ICAC.

He said while there is no evidence of widespread or systemic corruption in the ACT, a strong integrity system was important since breaches of public trust were "uniquely corrosive to the community's confidence that parliaments exist to advance the public interest or the common good".

But he said its reach when it came to politicians should be "clearly delineated and narrow".

It should not impinge on the strong protections for things said and done in parliament.

Parliamentary sitting days take precedence over court hearings for politicians, if they clash. Anything said in parliament or in its committees is protected - no-one can sue for defamation, and courts can't use whatever is said as a basis for prosecutions.

Politicians can't be arrested in the building and search warrants can't be carried out without the permission of the speaker.

"Parliamentary privilege protects the integrity and functionality of a parliament by protecting its proceedings from external impeachment or interference," Mr Duncan said.

"Any proposal for integrity arrangements that seeks to limit the Assembly's powers, immunities and privileges should be treated with an abundance of caution," Mr Duncan said.

The ACT parliament's ethics adviser, Stephen Skehill, has also urged limits on the powers of a corruption commission, saying it should only hear complaints as a last resort where other bodies couldn't or hadn't done the job.

Mr Skehill said he knew of no evidence that the ACT was "blighted by any widespread or systemic corruption or want of integrity" in public administration, but it would be exceptionally naive to believe that undetected corruption or integrity failures did not exist or could not exist.

A commission should be truly independent - appointed by the Assembly rather than the government, and led by someone with the respect and cooperation of all levels of the ACT administration.

It should hold hearings in private, so the public was not discouraged from coming forward, he said. Public hearings also risked damaging the reputation of innocent people, through the complaint itself or through accusations from people defending an accused person.

"In my view, at this stage the better course would be that the presumption of innocence should prevail and investigation should remain behind closed doors unless and until criminal charges are laid in a court," he said.

Mr Skehill said the commission should cover public servants, contractors, and people working in honorary roles, as well as the political sphere and the judiciary.

It should be a clearing house for complaints, and refer them on to the right bodies. It should only investigate complaints where no one else had responsibility or where they were not being investigated "as effectively or strenuously" as they should be.

The Assembly inquiry, chaired by the Greens' Shane Rattenbury, is considering questions such as whether the commission should have the power to investigate police, and what powers it should have to compel, issue search warrants and conduct sting operations.