A senator has rejected former prime minister John Howard's comments that women will never be equally represented in Parliament because of their greater roles as carers, by announcing she is pregnant and is "perfectly able" to continue in her position.

The comments, made on Monday night's Q&A, came as another panellist was questioned by her brother about her party's plan to hold a same-sex marriage plebiscite.

During a question about ethnic diversity in parliament, Greens senator Larissa Waters and diversity advocate Dai Le discussed the low representation of women in politics.

Senator Waters singled out recent comments made by former prime minister John Howard that women were unlikely to gain equal representation because they played a greater caring role than men.

"[Female representation] is at about one third," Ms Waters said.

"It's the lowest in the Liberal Party it's been for almost two decades, which is a very poor outcome.

"It's no surprise you have people like the former prime minister John Howard saying that women's capacity to work is inevitably limited by our caring responsibilities.

"No, it's not inevitable. That's just a factor of gender inequality that we can all work to change.

"If we have men stepping up and doing more work in the home, we women can share those duties more.

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"Just a word to John Howard, I'm about to have my second child, I have a few months to go yet — I wasn't probably meant to announce that on national television anyway — but I feel perfectly able to continue in my role.

"I don't feel like my capacity to do my job is in any way limited by that. Let's hope those old-fashioned views go by the wayside and we empower young people and particularly young women to enter politics and to enter the workplace with the equality they deserve."

Ms Le said she agreed with Mr Howard to the extent that she believed politics was more challenging for women.

"I think it's going to be a challenge to reach that 50 per cent — as a woman, as a mother, trying to balance, for me anyway, personally, to balance your family life in this political arena which is a really cut-throat, really male-dominated, really vicious, and the system is not very child-friendly," she said.

"I have to say to some extent I can, I agree with John Howard in the sense that it's challenging for women because they have to be the mother, the nurturer. I disagree with the fact it's unlikely. Anything is possible."

Gun advocate and National Party senator for Victoria Bridget McKenzie then pointed out she was breaking new ground for women in her party.

Ms McKenzie: "My party in Victoria turned 100 this year, happy birthday, Victorian Nationals. And I am the very first female issuer in the Senate or in the house that we've sent to Canberra in 100 years. Q&A host Tony Jones: "Is that why you carry a gun?" Ms McKenzie: "That's how I got there, Tony. That's how I got there."

Gay brother questions McKenzie over plebiscite

Alastair McKenzie questioned his sister Bridget about the same-sex marriage plebiscite. ( ABC News: Q&A )

Later, Ms McKenzie's brother asked via video about the Coalition's planned same-sex marriage plebiscite.

"Given the majority of parliamentarians and Australians now support marriage equality, is a $160-million plebiscite necessary and what impact do you think the debate will have on the emotional wellbeing of gay and lesbians like me?" he asked.

Ms McKenzie responded by saying it would only be damaging if Australians chose not to respect each other's views.

She added that she believed same-sex marriage was a "conscience issue".

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"I believe it is a conscience issue and it is that the definition of marriage is between a man and a woman," she said.

"Polls would suggest I'm in the minority."

However, she said a plebiscite would give everyone a conscience vote, and when pressed, she said she believed it should be binding.

"What I think is a way forward on this issue for MPs and senators like myself who have this conflict and tension is that it is a binding plebiscite," she said.

"That's my personal view. I don't know what Cabinet is deciding, but that's a view I believe will resolve the tension."

Senator Waters and Labor senator Doug Cameron, who was also on the panel, said politicians should simply "do our day job as politicians and deal with this issue how all these other issues have been dealt with in the past and vote for it in Parliament".

But co-panellist, Spectator Australia editor Rowan Dean, rejected the pair's claim.

"Gay men and women of Australia, these people you think who are your friends in the Labor Party and the Greens, they are not your friends, they are torpedoing your chance to have the marriage equality," he said.

"Every gay person in Australia should be on the phone to Malcolm Turnbull saying: 'We want the plebiscite'."