And the commission, which began hearings on Monday, says it has well-established mechanisms to deal with perceptions of bias or conflict of interest.

Dozens of disability advocacy organisations have called on commissioners John Ryan and Barbara Bennett to step down.

Both are former bureaucrats who oversaw parts of the disability system, and Mr Ryan was also the NSW shadow minister for the sector.

Labor's spokesman for disability issues Bill Shorten says the calls for them to step aside are understandable.

"If they've got specialist insight, they should be witnesses," he told Sky News on Monday.

"We're a country of over 24 million people - are we really saying that these were the only two people who could hear this royal commission?"

Royal commission chair Ronald Sackville acknowledged the controversy but said there was nothing unusual in such inquiries having to manage perceptions of conflict of interest or apprehended bias.

"Indeed, it is almost inevitable that if governments choose commissioners because of their experience and expertise in a particular area under investigation, perceptions of potential conflicts of interest or apprehensions of bias may arise," he told the first public sitting in Brisbane.

The commission says no more than three or four of the seven commissioners would participate in any particular public hearing, and the chair will not let anyone sit on a hearing if there were perceptions of conflicts or bias.

As well, only commissioners who participate in hearings will contribute to reports based on the evidence given.

Mr Sackville said the overriding consideration for the conduct of private sessions was the safety, security and comfort of those individuals.

"Under no circumstances will anyone be asked to tell their story to a commissioner with whom they do not feel comfortable."

Disability advocate Craig Wallace said the conflicts of interest in the cases of Mr Ryan and Ms Bennett were so profound they simply cannot be managed by established mechanisms.

He said he and others were not calling for an outright boycott.

"We are saying we feel unable to participate while these unmanageable conflicts remain," he said.

Mr Shorten urged anyone contemplating a boycott to stick with the process.

"I understand and support your concerns but this is a once in a lifetime opportunity to be heard and I would say please tell your stories," he said.

Australian Associated Press