Former Australian prime minister Tony Abbott has touched down in the US to deliver a speech for a conservative, pro-Christian organisation that funds legal battles around the world to oppose gay marriage and abortion.

Mr Abbott reportedly arrived in Los Angeles on a Qantas flight on Wednesday morning (AEDT), where he is expected to speak at an event organised by the Alliance Defending Freedom.

Some media outlets reported that the topic of Mr Abbott’s speech would be the importance of family; others that he will address marriage, which many US conservatives believe is ‘under attack’ as a result of the US Supreme Court’s legalisation of gay unions.

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The Alliance Defending Freedom (ADF) claims to have 3000 lawyers in the US and elsewhere that actively support its religious views.

“The legal system, which was built on a moral and Christian foundation, had been steadily moving against religious freedom, the sanctity of life, and marriage and family,” the ADF says on its website.

“And very few Christians were showing up in court to put up a fight.

“By funding cases, training lawyers and successfully advocating for freedom in court, Alliance Defending Freedom changed that.”

A spokesman said the former PM was invited because he was “an interesting fella”.

“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,” ADF media relations director Bob Trent told AAP on Tuesday.

“It’s a private gathering and Tony Abbott will be there.”

Mr Abbott’s sister Christine Forster, who identifies as a lesbian, said his decision to address the event was “disappointing”.

“It doesn’t sit with my understanding of how he views the world, so yes, I was surprised,” Ms Forster told Fairfax Media on Tuesday.

Prime Minister Malcolm Turnbull defended his predecessor’s right to address the lobby group earlier in the week, while also noting that he disagreed with the group’s views.

“There are, you know, fellow members of the Coalition who have different views and they are entitled to express them,” Mr Turnbull told Channel Ten.

“I respect their right to do so, just as they would respect my right to disagree with them.”

In a short statement to the ABC, an ADF spokesman said that Mr Abbott “has been a strong advocate for marriage and freedom throughout his career and has graciously agreed to address our private event”.

“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 of children, who flourish when society honours and promotes the roles of both mothers and fathers in children’s lives,” the statement said.

Mr Abbott announced on Saturday that he would recontest his Sydney seat of Warringah at the next federal election.

– with ABC and AAP