CANBERRA, AUSTRALIA - AUGUST 11: Prime Minister Tony Abbott during House of Representatives question time at Parliament House on August 11, 2015 in Canberra, Australia. Tony Smith was elected Speaker on 10, August following the resignation of Bronwyn Bishop. (Photo by Stefan Postles/Getty Images)

Australia’s ex-Prime Minister Tony Abbott stunned the country with a new jibe aimed at his gay sister.

As the country prepares for a controversial postal vote on same-sex marriage, right-wing former Prime Minister Tony Abbott has become a prominent voice against equality.

Mr Abbott, who led the country until he wad deposed in an internal party coup in 2015, has called on Australians to vote against equality against the wishes of his own sister Christine Forster, who is waiting for the right to marry her same-sex partner.

The pair have traded jibes across the campaign – but in a shocking radio interview today, Mr Abbott got personal about his sister’s family.

Ms Forster has six children with her fiancée Virginia.

Speaking to 2GB host Ben Fordham, Mr Abbott said of his opposition to same-sex marriage: “My position on this is not driven by religion, it’s driven by the fact that marriage is what produces families, families are what produces communities and societies and nations.

“I want to strengthen the family, I want to support the traditional families because it’s better for kids, if possible, to have a mother and a father.

“As everyone knows, my distinguished and much-loved sister Chris is on the other side of this argument. Chris has been a very good mother with her partner Virginia, they do a good job, but nevertheless I’m old fashioned enough to think kids do best with a mother and a father.”

He added: “Certainly, we all need male and female role models in our lives and normally the best male and female role models in our lives are our mother and a father.”

Mr Fordham asked if the politician’s relationship with his sister had broken down, after she accused him of “avoiding her calls”.

Mr Abbott said: “Chris has made it very clear that as a family we can all get on even though we don’t always agree on everything.”

However, Mr Abbott spurned his sister’s offer of a one-to-one debate on the issue.

He said: “There’s nothing exhibitionistic about the Abbott family. We are not show-offs, we do not parade ourselves or make a public spectacle of ourselves.

“I just think the last thing you want is a family squabble in public. I’m voting no [to a debate].”

Ms Forster, who frequently calls out her brother on Twitter, has not responded to the jibe.

What the hell is going on with same-sex marriage in Australia?

Labor Senator Penny Wong had won praise earlier last month for a powerful speech explaining why a public vote would invite attacks on the children of gay parents.

Of the suggestion that a public vote could provide a unifying moment for the country, Ms Wong said: “The Australian Christian Lobby described our children as the stolen generation.

“We love our children. And I object, as do every person who cares about children, and as do all those couples in this country, same-sex couples who have kids, to be told our children are a stolen generation.

“You talk about unifying moments? It is not a unifying moment. It is exposing our children to that kind of hatred.”

Mr Abbott recently stirred controversy when he visited US-based extremist lobbying group Alliance Defending Freedom (ADF) – despite the group allegedly pressuring countries around the world to keep sodomy laws banning gay sex.

The ex-PM pushed ahead with his plans to speak to the group, which has also opposed LGBT people in the military, same-sex adoption and equal marriage.

Related: Gay sister of Australia’s ex-PM Tony Abbott shuts down his anti-gay marriage stance