The Royal Commission of Inquiry into Abuse in Care has acknowledged a convicted child sex offender was at gatherings alongside people who are survivors of abuse.
Survivor Advisory Group sponsor Paul Gibson said the person was a partner of one of the advisory group members and had historical child sex convictions.
He said the commission was aware of the convictions in May but the nature of those was only disclosed to a commission employee on 22 August and the commission "immediately took action".
"At no point did the person in question participate in an official Survivor Advisory Group meeting,"
The man was at gatherings around the meetings because he was with his partner, Gibson said.
Gibson said there was no formal screening done before appointing members - nor of their partners - because the survivor advisory group are not employees of the Royal Commission.
But there would now be more rigourous due diligence of advisory group members.
"We've put in place processes so this person wouldn't be at gatherings in the future.
"We've got new people managing the Survivor Advisory Group.
"It was distressing for people to find out there was somebody with this history in the room, and we have learnt from that, and we want to acknowledge that.
Advisory group member Jane Stevens told Morning Report she was staggered the group was not informed and was feeling physically sick at the revelation.
"That is probably one of the worst possible scenarios for people who've been through abuse. I'm horrified," she said.
"I don't believe it's good enough for it to be treated as a learning, because I do feel re-victimised."
"I'm absolutely shocked that we've known nothing about this and I hear it on the radio. It's not good enough."
Gibson said it was the nature of the commission's work that it will be dealing with many people who had convictions.
"We're not going to make this mistake again, he said. "But we are going to continue to engage with hard-to-reach communities and we will transform how we care in the future. There have been mistakes made by us, we are learning."
But Stevens said the commission had to be very clear about the type of convictions.
"None of us are squeaky clean in that respect. I have historic convictions in the past, not serious ones, but as a result very directly of the abuse I suffered as a young person. I don't think you can cut people out totally because of historic convictions, but you have to be really clear in terms of the type of those convictions."
National Party leader Simon Bridges said it was impossible to comprehend how a convicted sexual offender was allowed anywhere near meetings which included survivors of sexual abuse.
He said Internal Affairs Minister Tracy Martin should front up about when she found out about this and what action she took.
Since its establishment, the Royal Commission has been heavily criticised for how it is operating. That includes appointing a gang member into a key role, using survivors for trial or pilot interviews, claims Sir Anand fell asleep while a survivor told their story and accusations commissioners shut down questions on potential conflicts of interest.
Inquiry chair Sir Anand Satyanand announced last month he was stepping down from the job.