Former National minister Anne Tolley has admitted telling her sister about Winston Peters' pension overpayment while she was "tired" and "cross" in a conversation she regretted.
She said her "outburst" was after media had already been leaked the information.
The Opposition MP was in the witness box at the High Court in Auckland on Thursday morning to defend herself against claims she breached Mr Peters' privacy in 2017.
The High Court action relates to a leak in 2017 revealing Mr Peters was overpaid his superannuation by nearly $18,000 over seven years, and had paid it back.
Ms Tolley - as Minister for Social Development - had been briefed by the ministry's then-chief Brendan Boyle and was questioned in court about who she had then told.
She admitted that several weeks after that briefing she mentioned the overpayment to her sister, remarking that Mr Peters was "not as great as she thought".
Ms Tolley said the comment was in passing and she did not provide much detail.
"I sincerely regret my outburst with my sister. I was tired and I was cross," she told the court.
Mr Peters' lawyer Brian Henry asked: "So you're saying you're not trustworthy?"
"I wasn't in that circumstance. I regret it," Ms Tolley replied.
Ms Tolley pointed out that reporters at two newsrooms had already been tipped off by an anonymous source at that time.
She said she did not make the anonymous calls and did not arrange for them to be made.
"I have no knowledge of who did make those phone calls."
The MP revealed she also told her husband, her senior adviser and the prime minister's chief of staff, and trusted them all to keep the information secret.
She said she was "disappointed" her adviser - whose name is suppressed - then told other people in her office against her express instructions not to do so.
Ms Tolley said, however, she was "absolutely confident" none of the people told were responsible for the leak as they did not know details such as how much Mr Peters was overpaid.
Under questioning from Mr Henry, Ms Tolley said she wished MSD had never informed her of the matter in the first place.
"I wish they hadn't, because then I wouldn't be sitting here."
She said she relied on Mr Boyle to decide whether a briefing under the "no surprises" convention was appropriate.
Asked what she could do with the information she was told, Ms Tolley replied: "Nothing."
Paula Bennett's turn
National deputy Paula Bennett also gave evidence on Thursday.
She told the court she was briefed under the "no surprises" convention by State Services Commissioner Peter Hughes and his deputy Debbie Power in early August.
She later mentioned the overpayment to then-Prime Minister Bill English and colleague Steven Joyce, but not in great detail, she said.
Both men had enquired about her well-being given news of an upcoming "scandal" being teased online and out of concern it might relate to false claims about her on social media.
The conversations came after media had already received anonymous tip-off, she said.
Asked about commentary that the National Party was responsible for the leak, Mrs Bennett dismissed the "allegations".
"I have heard so many different versions of who it might be and I see no more validity in that than half of the rumours I hear."