Gillian Triggs has labelled Scott Morrison’s response to the Bourke Street attack “foolish” and “unhelpful”, and called for companies to do better on sexual harassment complaints after Michelle Guthrie’s explosive claims against the former ABC chair Justin Milne.

On Monday night’s Q&A panel show, Triggs directly referenced Guthrie’s allegations, revealed on Four Corners only minutes earlier.

On the same program, the former Nationals leader John Anderson warned Barnaby Joyce off having another run at the party’s leadership, and insults flew as Labor’s Chris Bowen called Joyce “the most overrated politician in Australia” and a questioner called the Australian Capital Territory Liberal senator Zed Seselja a “bedwetter” for supporting the toppling of Malcolm Turnbull.

But Triggs was the first panellist to respond to the breaking news that the former ABC managing director Guthrie had alleged that Milne touched her inappropriately.

Milne has denied the allegations, and Guthrie did not detail whether the harassment was sexual, saying it “felt icky … was unprofessional and inappropriate”.

Milne told the ABC’s Four Corners earlier on Monday: “I never, ever behaved in any inappropriate way with Michelle. I had no reason to whatsoever and I didn’t.

“I’ve had no physical relationship with Michelle at all. I never, ever acted inappropriately with Michelle, or indeed with any other woman in the workforce, or any other woman at all.”

Triggs, the former president of the Human Rights Commission, said companies and corporate boards needed to adopt a more stringent, fairer way of handling harassment complaints.

At the commission, she said, 76% of matters were resolved quietly and confidentially.

“Once you bring it to the public arena you inevitably lose your ability to settle the matter,” she said. “Women inevitably come off second or third best. Women tend to be victimised twice.”

“The woman concerned will raise the issue at particular levels, perhaps test the waters, see what kind of response she’s getting, and very rapidly retreat and not make a formal complaint.

“Women simply don’t proceed because they see it as too dangerous, they know there will be a very high probability that they will be the ones that suffer … We need to understand that we can’t solve this problem in the public arena.”

In a response to a questioner, Triggs also criticised the prime minister, Scott Morrison, for his response to the Bourke Street attack and his dismissive attitude to mental illness.

It emerged on Sunday that Hassan Khalif Shire Ali believed he was being chased by “unseen people with spears” and was abusing substances in the lead-up to Friday’s fatal terrorist attack.

On Monday Morrison said referencing these delusions was “a lame excuse”. Terrorism experts and lawyers responded, saying his comments were “simplistic” and that better mental health access could prevent violence.

“The perpetrator intended to terrorise, that meets the definition of terrorism,” Triggs said. “[But] so many of these acts are by people with profound mental disturbance or substance abuse. I think to ignore that reality is very foolish indeed.

“It is impossible for security agencies to predict when people will go off the rails in the way we have seen recently. To ignore this phenomenon and to say it is purely an ideological terrorist attack is an unhelpful comment because it diverts us from those kinds of inquiries that we need to make … It might be that we can help them avoid these great extremes of paranoid behaviour when they have these episodes.”

But Seselja defended the prime minister, saying he was right to “call [terrorism] out for what it is”.

Later in the night, the program hit a lighter note when discussing Joyce.

Anderson, who led the National party from 1999 to 2005, warned Joyce not to push for the leadership before the next election.

“What has happened has happened,” he said. “I think it’s incredibly important that we not send the message that white-anting of leaders has become part of the political norm in Australia.”

He did not rule out a return in future. “You’ve earned your spurs once,” Anderson told his former colleague. “You’ve lost them. Go and rebuild them. It’s not going to happen overnight.”

Labor’s shadow treasurer, Chris Bowen, was not as kind. “Barnaby Joyce is in my opinion the most overrated politician in Australia,” he said, to laughs and applause from the crowd.

“What, three months after he lost the leadership we’re talking about him coming back? That just underlines how dysfunctional this show is.”

Would a Barnaby Joyce resurgence drive a dagger further into the Government’s chances of winning the next election? #QandA pic.twitter.com/qAua7eRrCG — ABC Q&A (@QandA) November 12, 2018

Bowen’s response also drew an appreciative laugh from the question asker, who in no uncertain terms accused Joyce of “placing a dagger” into the government and called Seselja a “bedwetter”.

The senator interpreted this to mean he was being accused of “slavishly following” the polls when his party rolled Turnbull.

Earlier in the night, Seselja defended the decision to change leader and maintained that switching to Morrison was important to hold on to the Liberals’ “base”.

“The reason I came to the conclusion I did was that we as a party and as a government had lost our connection with a pretty significant chunk of our supporter base,” he said. “While it is often said that politics is won in the centre, you can’t reach out to the swing ground without a strong supporter base.”