The ABC board has defended its chair Ita Buttrose from a “disrespectful” attack by Liberal powerbroker Michael Kroger, who labelled her leadership of the public broadcaster a “terrible failure”.

Kroger’s vitriolic spray, broadcast by Sky News Australia, was so personal it forced ABC board member Joseph Gersh to mount a public defence of the media veteran on ABC radio in Melbourne.

Kroger was reacting to news that former attorney general Christian Porter had dropped his defamation case against the ABC without securing an apology or retraction from the public broadcaster.

In the extraordinary spray Kroger said Buttrose was a “hopeless failure” and should resign because some ABC programs were so biased they were throwing “political acid” in the Liberal party’s face.

“Ita should resign,’’ Kroger, a former board member of the ABC, told Chris Kenny on Sky.

“Ita has been a terrible failure. And I know there are many people in the Coalition, including people in the cabinet who regret her appointment.”

Gersh said he was unaware of any members of the Coalition who were critical of Buttrose; and she had a professional working relationship with the Morrison government which appointed her as a captain’s pick in early 2019.

However, in the Coalition party room on Tuesday, the Nationals MP George Christensen complained of systemic bias at the public broadcaster and urged the government to “strike while the iron is hot ... starting with the chairman”, a comment that Coalition members believe was targeted at Buttrose.

After the meeting, the Nationals MP Barnaby Joyce backed Christensen’s comments, arguing that Coalition members were concerned the ABC “more and more sees its remit as reflecting one side of politics”.

Gersh said Kroger was “simply out of line” for attacking the media veteran who had decades of experience and service.

Asked by Virginia Trioli whether the board regretted the broadcaster’s coverage of Porter, Gersh said the board was by definition at arm’s length from editorial and it was not his call.

“The ABC has to be frank and fearless in what it does in holding those in power to account and has to do so consistently within its charter and within its editorial policies, and the board can’t interfere,” Gersh said.

Kroger claimed the ABC staff were “more powerful than ever”. “They have weaponised Four Corners, Q+A in particular, which is like political acid in the face of the Liberal party,’’ he said.

“It’s a shockingly biased program. The Drum, these current affairs shows are just weaponised against the Coalition.”

Kenny agreed with Kroger, adding that the rape allegation against Porter, that he denies, was “disgusting stuff to put into the public arena.”

Buttrose, 79, has proved a strong defender of the ABC’s independence, labelling claims of bias “malicious garbage” and asking why “some individuals and some commercial media outlets [were] campaigning loudly for its demise”.

Last year she accused the government of a pattern of behaviour which “smacks of political interference” after the Coalition complained about the Four Corners program Inside the Canberra Bubble, which raised questions about government ministers’ alleged sexist and inappropriate behaviour towards women.

The dispute comes as officials from the Department of Industry, Science, Energy and Resources were asked on Thursday by the Labor senator Murray Watt at a Senate estimates hearing whether any staff had “expressed discomfort or an unwillingness to meet with minister Porter since his appointment”.

Luise McCulloch, a department deputy secretary, said the department was aware of one “complaint” that was being dealt with through “normal workplace safety policies and support for staff”.

Watt then asked: “So, this is one complaint, is that the right word?”

“It was one issue raised by one person in relation to dealing with the minister,” McCulloch replied.

According to the 2021-22 budget statement, the department has an average of 3,096 staff.

The department declined to answer further questions on whether a reason was given for not wanting to meet Porter, the timing of the complaint, or whether the complainant was a man or woman, on confidentiality grounds.

Later on Thursday, the department secretary, David Fredericks, said the complainant was in a role that would interact with the minister “quite rarely, if at all”.

Kylie Bryant, the department’s chief operating officer said it was a written complaint made to human resources that did not require an investigation, and the matter had been resolved. She had informed Porter’s chief-of-staff, but did not know if Porter had been told.

Guardian Australia contacted Porter for comment.

Adam Morton and Paul Karp contributed to this report