New figures have revealed the potential impact on the Turnbull government of inaction on same-sex marriage before the next election, with one analyst suggesting they have serious electoral implications.

As the Coalition prepares for a party room showdown to debate its policy on marriage, a TND analysis found hundreds of voters with a direct stake in the issue across the 20 most marginal government-held electorates.

The figures showed there was an average of 409 Australians who said they were in a de facto same-sex relationship across these seats, according to 2016 census data.

That included 419 people in the seat of Forde, held by 1062 votes, 430 in Capricornia, where the margin is 1111 and 430 in Gilmore (1503-vote margin).

With most polls predicting a swing against the government, one political expert said the figures had serious “electoral implications” for the Coalition, which currently holds power with a one-seat majority.

The latest Newspoll released on Sunday night had Labor still ahead at 53 to the Coalition’s 47 on two-party terms – the 17th poll in a row predicting resounding defeat for the government.

“Without these seats, the Coalition loses government,” Zareh Ghazarian, a political scientist at Monash University, told The New Daily.

“If you were to say all these people, who are directly affected by the government’s inaction on marriage equality, would definitely vote against the government, then they are losing large parts of the margin that they won some of these seats by.”

Elsewhere, in the country’s most marginal seat, Herbert, in Queensland, which is held by Labor by only 37 votes, there were 524 Australians who identified as in a same-sex relationship, while in the Labor seat of Melbourne Ports (2337-vote margin), the number was 1757.

Malcolm Turnbull has recalled his MPs to Canberra one day earlier amid divisions of same-sex marriage. Photo: AAP

However, others were less convinced that same-sex marriage could have a material impact on the election given the data.

“If anything, I think the geographical spread of same-sex couples tends to diminish its importance in terms of marginal seats,” electoral analyst William Bowe said.

Describing the numbers as “modest”, Mr Bowe said same-sex couples were usually concentrated in the inner-cities where the Liberals already struggled.

“One exception is Trevor Evans in the seat of Brisbane, which is certainly loseable for the LNP,” he said.

“It’s probably a good thing for the party that they have this member in this seat.”

The openly gay politician, who holds his seat by more than 11,000 votes, is one of five Liberal MPs behind a push to have a free vote on marriage in Parliament this week. There are 1503 potential voters in a gay couple in his seat.

Victorian Labor assistant secretary Kosmos Samaras said the numbers were “still pretty significant” in marginal seats where the results would be tight.

“If the Coalition is wanting to defend those seats, it will have to find additional votes elsewhere, because these people won’t be voting for them in the current climate,” he said.

Ahead of a special party meeting to discuss the issue on Monday, Liberal Senator Dean Smith revealed his same-sex marriage bill would include exemptions for religious figures and celebrants who did not wish to perform gay marriage ceremonies.

Three lower house Coalition MPs would have to cross the floor in order for the legislation to be put to a vote, and it is believed Mr Evans, as well as Tim Smith, Warren Enstch and Trent Zimmerman, are considering this option.

Mr Turnbull last week reiterated the government’s position of holding a plebiscite.

“The only reason that a plebiscite or vote has not been held, the only reason every Australian has not been given the opportunity to vote on this is because of Bill Shorten’s opposition,” he said.

Senior government conservatives have advocated for a non-binding postal plebiscite, but LGBTI groups revealed legal advice on Sunday that suggested such a move would be blocked by the courts.

The numbers – Coalition’s most marginal seats

1. Forde (Qld), margin 0.6% (1062 votes), voters in a gay couple 419
2. Capricornia (Qld), margin 0.6% (1111 votes), voters in a gay couple 257
3. Gilmore (NSW), margin 0.7% (1503 votes), voters in a gay couple 430
4. Flynn (Qld), margin 1% (1814 votes), voters in a gay couple 253
5. Robertson (NSW), margin 1.1% (2179 votes), voters in a gay couple 452
6. Chisholm (Vic), margin 1.2% (2154 votes), voters in a gay couple 317
7. Dunkley (Vic), margin 1.4% (2557 votes), voters in a gay couple 392
8. Banks (NSW), margin (1.4%) (2588 votes), voters in a gay couple 337
9. La Trobe (Vic), margin 1.5% (2701 votes), voters in a gay couple 357
10. Dickson (Qld), margin 1.6% (2911 votes), voters in a gay couple 479
11. Petrie (Qld), margin 1.6% (3059 votes), voters in a gay couple 543
12. Hasluck (WA), margin 2.1% (3337 votes), voters in a gay couple 323
13. Page (NSW), margin 2.3% (4822 votes), voters in a gay couple 471
14. Corangamite (Vic), margin 3.1% (6165 votes), voters in a gay couple 423
15. Dawson (Qld), margin 3.3% (6034 votes), voters in a gay couple 265
16. Bonner (Qld), margin 3.4% (6095 votes), voters in a gay couple 447
17. Boothby (SA), margin 3.5% (6672 votes), voters in a gay couple 366
18. Swan (WA), margin 3.6% (5848 votes), voters in a gay couple 570
19. Pearce (Wa), margin 3.6% (6,312 votes), voters in a gay couple 420
20. Leichhardt (Qld), margin 4% (7022), voters in a gay couple 677

* Includes Coalition vs Labor seats only. Data: ABS, AEC