A senior Liberal opposed to legalising same-sex marriage is using an analysis of the religious and ethnic makeup of key marginal seats to warn colleagues against change.

New South Wales Senator Concetta Fierravanti-Wells referred to the analysis during a speech to the marathon six-hour Coalition party-room meeting on the issue yesterday.

Senator Fierravanti-Wells told colleagues that she believed opinion polls showing majority support for legalising same-sex marriage did not reflect the views of a "silent majority" of Australian voters.

She said changing the marriage laws - or being seen to condone change - would cost the Coalition seats at the next election.

The senator referred to an analysis she had undertaken which identified marginal seats with high percentages of religious voters.

A copy of the analysis, dated July 3, lists 14 seats across New South Wales, Queensland, Victoria and Tasmania with relatively high proportions of Catholic, Orthodox, Anglican, Muslim or Buddhist voters or overseas-born voters from ethnic communities likely to oppose same-sex marriage.

In a written foreword to her analysis, Senator Fierravanti-Wells, the parliamentary secretary for social services, also responsible for multicultural affairs, says she believes there is strong opposition among culturally and religiously diverse communities to changing the marriage laws.

"If it is made a priority it would raise strong opposition to the faith of many Australians, including our migrant communities," she writes.

She notes that faith leaders from across Australia had written to the Government in June, objecting to any change.

Her analysis includes the Sydney seat of Barton, the Liberals' most marginal seat, held by Liberal Nickolas Varvaris on 50.31 per cent.

She says Barton has nearly eight times the proportion of eastern Orthodox constituents than the national average, four times the proportion of Muslims, a higher-than-average Greek population and fewer who said they had "no religion".

Other western Sydney marginal seats included are Reid, Werriwa, Banks and Parramatta.

The analysis says Parramatta, held by Labor's Julie Owens on 50.57 per cent, is 25 per cent Catholic, has 10 times the national average of voters identifying as Hindu, four times the rate identifying as Islamic and higher-than-average percentages of those born in India and Lebanon.

Other seats examined include the nation's most marginal seat, the Labor-held seat of McEwen in Victoria, the Queensland seat of Lilley held by former treasurer Wayne Swan and the next most marginal Liberal-held seats of Petrie in Queensland, and Eden-Monaro and Dobell, in NSW. It also examines Capricornia, in Queensland, Lyons, in Tasmania, and Chisholm and Bruce, in Victoria.

Today, Senator Fierravanti-Wells confirmed she had undertaken the analysis.

"We must not ignore the makeup of the silent majority of mainstream Australia where faith, marriage and families are bedrock issues," she told me. "The silent majority includes our multicultural and multi-faith communities for which marriage and family values are very important and are strongly supported."

Yesterday, a majority of Coalition MPs and senators decided not to allow themselves a conscience vote when a cross-party private-members' bill to legalise same-sex marriage comes before the Parliament.

Instead they will be expected to vote against it, in line with party policy.

Karen Middleton is a freelance journalist and long-time member of the parliamentary Press Gallery.