Peter Westmore is president of the National Civic Council in Australia.

The head of the leading Australian Catholic-based lobby group campaigning against same-sex marriage has attended and blessed his daughter's gay wedding in New Zealand.

Peter Westmore is the successor to the high-profile B. A. Santamaria as president of the National Civic Council (NCC), a conservative group that for decades has sought to shape Australian public opinion and policy on key social and political issues, including on same-sex marriage.

The NCC is one of the religious groups opposed to Liberal Democrat senator David Leyonhjelm's "freedom to marry" bill, scheduled for debate in the Australian Federal Parliament on Thursday. The NCC is encouraging supporters to use their email accounts to lobby MPs against the bill.

An important political influence on Prime Minister Tony Abbott, the NCC also organised the "Save Tony [Abbott]" email campaign during last month's spill motion over the Government's leadership.

A core NCC policy is that it "rejects lifestyles which undermine family values".

Westmore has railed against Australian and international moves toward same-sex marriage. In 2012 he told an audience at the National Marriage Day rally outside Federal Parliament in Canberra: "We are here to celebrate marriage, the union between a man and a woman, because it provides the foundation on which every successful society, from time immemorial, has been based."

In 2013 he questioned same-sex marriage legalisation in New Zealand, and in 2014 opposed a Greens' bill for Australian recognition of foreign same-sex marriages.

"The battle to defend the true meaning of marriage is continuing," he wrote in June last year.

Yet on Tuesday he confirmed that in November he attended his daughter Trish's marriage to partner Christy Hackney in New Zealand, where same-sex marriage was made legal in 2013.

He said his daughter's wedding had his blessing: "She always has my blessing in everything she does."

Asked on Tuesday if he still supported the NCC's opposition on same-sex marriage he said: "Of course I do."

Westmore did not respond when asked about the apparent hypocrisy in his personal and political positions on the issue. He asked Fairfax Media to "respect" his family's privacy.

Australian Marriage Equality national director Rodney Croome said the revelation of Westmore's support for his daughter's wedding was a reminder that marriage equality affected Australian families across the political spectrum.

"Some people will call Mr Westmore hypocritical for opposing marriage equality, but my hope is that, like many other people of faith, he's on a journey towards supporting it," Croome said.

Liberal MPs and senators have been inundated with emails arguing for and against the freedom to marry bill, with pressure growing from marriage-equality advocates that Liberal MPs be given a free vote on the matter.

Labor has already decided to allow a conscience vote on any such bill.

So far only a small group of Liberal MPs, including frontbenchers Malcolm Turnbull, Simon Birmingham and Kelly O'Dwyer, have come out publicly in favour of same-sex marriage.

But marriage-equality advocates, who have been meeting with MPs over recent weeks, say a significant change is under way inside the party, claiming 11 Liberal MPs since January have privately switched to backing reform.

Abbott is a strong supporter of maintaining the status-quo definition of marriage.

Supporters of same-sex marriage within Parliament are optimistic that if Liberal MPs were allowed a free vote, Senator Leyonhjelm's bill will pass the Senate with a slim majority.

There is less hope that same-sex marriage would pass the Lower House.

Westmore's daughter Trish Hackney-Westmore - a married name after the November wedding to Hackney - did not return calls.

In 1957 the NCC grew out of the anti-communist Catholic Social Studies Movement also known as 'The Movement'. The Movement was a key player in the split of the Australian Labor Party in mid-1950s and the formation of the Catholic-based Democtratic Labor Party.