New Zealand’s troubled royal commission into abuse in state care has been dealt another blow, with commissioners revealing survivors unwittingly spent time with a convicted child sex offender tangentially connected to the inquiry.

The royal commission is investigating historic abuse of children and adults by state-run institutions between 1950 and 1999. It is the largest and most complex royal commission ever undertaken in New Zealand, and has been plagued by issues since its inception, including the resignation of its chair, Sir Anand Satyanand, last month.

Now, survivors say they feel re-traumatised after it was revealed they unwittingly ate, drank and shared a motel with a man convicted of historical sex crimes against a child. The man was the partner of one of the survivor advisory group members, and drove her to meetings, and socialised with survivors outside official meetings.

A spokesperson for the royal commission said commissioners were made aware of the man’s criminal background on 22 August, but survivors who had unknowingly spent time with the offender were not informed until 24 September when media reports emerged.

“That is probably one of the worst possible scenarios for people who’ve been through abuse. I’m horrified,” said Jane Stevens, a survivor of abuse and an advisory group member.

Stevens told RNZ the revelation had left her feeling physically sick. “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.”

The royal commission spokesperson said that at no time did the man attend official survivors’ meetings, although survivors have said he did eat with them, and spent a night in a motel with them.

Commissioner Paul Gibson has said the nature of the inquiry meant many of the people it was dealing with had criminal convictions.

A royal commission spokesperson said in a statement: “We acknowledge the distress that some survivors may feel about this but want to assure them that this won’t happen again.

“We have a new manager of the survivor advisory group who has begun the process of vetting the members.”

Tracey Martin, the internal affairs and children’s minister, has called an urgent meeting with the inquiry commissioners, and has refused to rule out sackings.

Justice minister Andrew Little said he was “stunned” to hear of what happened. “The reality is when you’re dealing with survivors of sexual abuse you don’t mix them up with a sexual abuser.”

Simon Bridges, the leader of the opposition National party, said the fiasco was “ an abhorrent and gross breach of trust by government”.

“This is one of the biggest inquiries in New Zealand history, it will cost $78m,” said Bridges. “The highly respected chair has resigned, there have been a range of other issues, and now this. How can anyone now have confidence in this royal commission? The minister must explain.”