• Christine Forster (right) with her brother Tony Abbott. (AAP)
Tony Abbott's sister has described his decision to address an anti-gay marriage, anti-abortion Christian lobby group in the US as 'disappointing'.
27 Jan 2016 - 8:17 AM  UPDATED 27 Jan 2016 - 11:57 AM

Tony Abbott's sister and same-sex marriage campaigner Christine Forster has expressed concern about his upcoming visit to anti-gay marriage and anti abortion Alliance Defending Freedom (ADF) group in the US this week.

"On the face of it, and without knowing what he is going there to say, it is disappointing," she told Fairfax media.

But she added she didn't think her brother was winding back rights for the gay community, and that she did not know enough about the alliance to pass much judgement. 

The former prime minister is due to deliver a speech about the "importance of family" to the conservative Alliance Defending Freedom group at an event it says is aligned with the United Nations in New York later this week.

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"Mr Abbott has been a strong advocate for marriage and freedom throughout his career and has graciously agreed to address our private event," an ADF spokesperson told SBS News on Tuesday.

The ADF, which opposes abortion and LGBT rights, was founded in 1994. Its CEO is Alan Sears, a former official for the justice department under former president Ronald Reagan’s administration.

ADF insists Abbott event is not 'intentionally secretive'


A spokesperson from the Alliance Defending Freedom said the event featuring Abbott's speech is not "intentionally secretive".

Mr Abbott arrived in Los Angeles on a Qantas flight on Tuesday (Wednesday morning AEDT).

"I don't know if it is intentionally secretive," ADF media relations director Bob Trent, discussing the event, told AAP on Tuesday.

"For us, it was never intended to be a public event.

"It was a dinner ADF was having and Tony Abbott aligned on some of these issues, and I think they thought he would be an interesting fella to hear from, so they invited him to come."

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Mr Abbott is also reportedly scheduled to meet News Corp executive chairman Rupert Murdoch in New York.

The ADF is surprised by the media interest in their event.

"It is going to be in New York, we have invited variety of people," Mr Trent said, declining to detail who would attend.

"It's a private gathering and Tony Abbott will be there."

The ADF boasts on its website of having more than 3000 allied lawyers to help win court victories in the US and around the world.

It describes itself as "an alliance-building legal organisation that advocates for the right of people to freely live out their faith".

"Along with our work to defend human rights such as free speech and religious freedom, ADF affirms the good of marriage and the value of strong families around the world, particularly on behalf children, who flourish when society honours and promotes the roles of both mothers and fathers in children's lives," the ADF said in a statement sent to AAP.

"Mr Abbott has been a strong advocate for marriage and freedom throughout his career and has graciously agreed to address our private event."