Tony Abbott's sister Christine Forster, the prominent same-sex marriage campaigner, says she was "disappointed" when she learned the former prime minister plans to address American anti-gay group, the Alliance Defending Freedom.

But she also insisted her brother was not in favour of winding back legal rights for the gay community  - a key plank of the Alliance's agenda - and said the group may be less radical than it is being portrayed.

Australian of the Year, David Morrison, who has pledged to pursue tolerance for all groups in society, and transgender Army officer Catherine McGregor, contacted by Fairfax Media, both declined to comment on Mr Abbott's intentions to address the Alliance in New York on Thursday with a speech entitled "the importance of family".

The Alliance says its New York conference is aimed at promoting the benefits of the traditional family to the United Nations but the group has been criticised for its work in trying to wind back "tolerance training" in schools, criminalising abortion and funding the legal defences of people wishing to discriminate against gay and lesbian people based on Christian beliefs.

Ms Forster told Fairfax Media: "On the face of it, and without knowing what he is going there to say, it is disappointing.

"It doesn't sit with my understanding of how he views the world, so, yes, I was surprised. I don't believe Tony is in any way winding back rights for the gay community but I was surprised to hear he was going."

Ms Forster, who has not spoken to Mr Abbott about his planned appearance, said she had never heard of the Alliance until Monday but said she had conflicting reports of whether the group was of the "loony far right" or could be compared to the Australian Christian Lobby.

"If it is the equivalent of the ACL, I would have no qualms in Tony going and speaking to them," she said.

Former US president George W. Bush and current Republican presidential candidate Marco Rubio, who spoke with Prime Minister Malcolm Turnbull during his recent US visit, have spoken to the Alliance, she said.

The Alliance runs a legal academy, which gives practicing attorneys free training in defending cases opposing same-sex marriage and abortion, and church/state issues. In return, participants must provide 450 hours of "pro bono/dedicated legal work on behalf of the Body of Christ".

Rodney Croome, the national director of Australian Marriage Equality said Mr Abbott had the right to speak to whichever group he wished. "I'm disappointed that he's going to speak to a group that goes out of its way to promote division," he said.

"I hope he tells this group that the country he was prime minister of believes above all else in inclusiveness, tolerance and equality."

Opposition Leader Bill Shorten said the Alliance represented "some of the worst excesses of the far religious right in America".

"Everything from their trenchant and radical opposition to the right to choose, right through to people talking about criminalising homosexuality. What Mr Abbott does is up to him, but he's the former Liberal prime minister of Australia. What worries me is that his views are commonly held within the right-wing of the Liberal Party," he said.

"Malcolm Turnbull would like everyone to believe he's the one person in the Liberal Party. The truth of the matter is that many Liberal MPs and the powerbrokers of the Liberal Party think like Mr Abbott and they've simply installed Malcolm Turnbull as the frontman."

On his first day as Australian of the Year, Mr Morrison, said he wanted to see same-sex marriage legalised sooner "rather than later" but had no comment on the Alliance speech.

"I won't make any comment about what Mr Abbott is doing and what he is saying. He is a proud Australian and a former PM of this country who I have come to know and I won't make comment about that," he said.

Appearing at the launch of a new Tourism Australia campaign in New York on Monday evening, Foreign Minister Julie Bishop said Mr Abbott's speaking engagements were "a matter for him". "He's a member of Parliament, he can make those decisions," she said.

Mr Abbott will be joined by two other notable speakers at the New York reception on Thursday: well-known conservative American activists Ryan Anderson and Sherif Girgis.

Mr Anderson is a young academic known for his pro-life views on abortion, arguments that Christian religious freedoms are under assault and particularly his criticism of same-sex marriage. He was highly critical of the US Supreme Court's decision to legalise gay marriage across the country and believes that children need both a mother and a father, often citing widely debunked research to support that view.

Mr Girgis co-authored a book with Mr Anderson called What is Marriage? Man and Woman: A Defense. The book argues that marriage is about procreation and that legalising gay marriage will undermine the institution, leading to polygamous unions.

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