The office of human services minister, Alan Tudge, mistakenly sent a journalist internal departmental briefings about a welfare recipient’s personal circumstances, which included additional detail on her relationship and tax history.

Senior departmental figures were grilled at Senate estimates on Thursday about the release of welfare recipient Andie Fox’s personal information last month.

Fox had written an opinion piece critical of Centrelink and its handling of her debt, which ran in Fairfax Media in February. The government released her personal details to Fairfax journalist Paul Malone, who subsequently published a piece attacking Fox and questioning the veracity of her claims.

Two responses were given to the journalist, one from the department of human services and the other from Tudge.

The department said its response – three dot points containing only minimal detail on Fox’s personal history – was cleared by lawyers and was lawful. The minister’s office then added two quotes from Tudge and sent its own response to Malone.

Centrelink fiasco: when can the government release your personal information? Read more

Guardian Australia can now reveal that the minister’s office also accidentally sent the journalist two internal briefing documents, marked “for official use only”, which had been prepared by the department.

Those documents contained additional information on Fox and her personal circumstances, which went beyond the dot points prepared by the department. They included further detail of her relationship history, including when she separated from her partner.

Those documents were then sent to Malone. The documents were also mistakenly sent to Guardian Australia when it raised questions about the disclosure of Fox’s personal information.

No mention of those documents was made in Senate estimates on Thursday, despite repeated questioning of what the minister had disclosed to Malone. Tudge’s office has now conceded the documents were sent to Malone in error. But the office says it was of no consequence, because all of their contents had been legally cleared by the department.

A welfare recipient’s personal details are considered protected information under social security law, and any unlawful disclosure is considered a criminal offence. Earlier, the department told estimates that social security law only allowed it to disclose the minimal amount of information needed to correct the public record.

Department of human services secretary, Kathryn Campbell, said Fox had made unfounded claims, that may have caused other welfare recipients to believe they were not complying with their own requirements. She said that made it necessary to release Fox’s personal information to correct the record.

“That’s why we felt that it was appropriate to release the information, so that people knew it was important to file their tax returns and tell us about changes in their circumstances,” Campbell said.

She rejected any claim that the department was targeting anyone who criticised Centrelink. “It’s about whether the people of Australia are going to continue to have confidence [in the system] ... so senator when we look at each case, first of all I ask, have we made a mistake?” Campbell said.

“We know that that sometimes happens, that’s our first instinct … that’s our first reaction,” she said.

The department’s chief legal counsel, Annette Musolino, said she was confident the department’s disclosure of information was lawful. She said it was allowed under a section of the social security (administration) act, which allows for disclosures for the purpose of “social security law”.

The department has interpreted that to mean it can release information to maintain confidence in and integrity of the social security system. It says that allows the department to release information to correct the public record.

Campbell used her opening statement to the committee to defend the “robo debt” system.

She said almost half the cases of inaccurate debts reported publicly were not raised under the automated system. Campbell conceded that 6,600 initial letters sent out to welfare recipients went to old addresses. But Campbell said the department was now using registered mail.